These people are essentially magical prisoners, and the magical effect is a kind of stasis. My PCs may at some point stumble on them, but of course, they may not. Apparent age is subjective age - I don't know how long they've been imprisoned, so 'apparent age' is basically 'number of years from birth until magical trapping'; equivalent age for the Demi-humans is pretty much what I reckon the developmental stage of the individual is compared to a human lifespan - which is what leads on to the second half of the post.
Female Cleric - Braeth Carellen - 7th Level: apparent age 23
Str 11 Int 14 Wis 16 Dex 7 Con 10 Cha 13
Content and systematic; she is concerned about the heretic toadmen in the salt-marshes
Female Dwarf - Eida Grimsilver - 10th Level: equivalent age 32 (age in years 128)
Str 15 Int 5 Wis 11 Dex 6 Con 15 Cha 8
Stereotypical, but insincere; she only has one eye
Male Elf - Taraneer Moonsword - 4th Level: equivalent age 19 (age in years - don't know: 19... 100... 1000?)
Str 13 Int 15 Wis 16 Dex 11 Con 10 Cha 12
Shrewd and selfish; has tattooed eyelids (a lion surrounded by petals on left lid chases a winged fish on the right)
Str 14 Int 5 Wis 11 Dex 9 Con 11 Cha 17
Alternately savage and fussy; he wants to trap a giant lizard
Str 13 Int 12 Wis 9 Dex 15 Con 8 Cha 12
Upbeat and enigmatic; he used to be a gladiator
Male Magic User - Howland Banroth - 6th Level: apparent age 36
Str 7 Int 15 Wis 8 Dex 12 Con 11 Cha 8
Both softly-spoken and fabulous; wears high shiny boots
Female Thief - Thealil Ember - 8th Level: apparent age 35
Str 7 Int 5 Wis 14 Dex 14 Con 5 Cha 13
Comes across as engaging and receptive; is a recovering addict
All of this raises questions. Taraneer Moonsword for example. Why 'Moonsword'? Is this a family name, a gang-type nickname or because he owned a special blade called the Moonsword? If the Moonsword is a thing, does Taraneer still have it, what are its powers and its value? And what (if anything) do his tattoos mean?
Will Ryon Crawn get to trap his giant lizard? Where? How?
What was Thealil Ember addicted to? Is she going to go off the rails now she's been freed?
Why is Braeth Carellen concerned about the toad-men? What is the nature of their heresy? Are they really so bad? And what have they been doing since she was imprisoned?
And that's another set of questions, regarding how long these people have been here. Days? Years? Millennia? Do they know, or will it be a shock? Is everyone they know dead? Has the Moonsword rusted, have the toad-men gone to hell with their heresy, are the giant lizards extinct? Or have they only been gone a matter of months or years, and can pick up the threads of their lives?
As for the ages of the Demihumans... I've never really had a settled way of working them out, instead just picking numbers that 'feel right'. I tend to assume something like Tolkienish lifespans for Halflings and Dwarves. I've been a Tolkien fan for longer than I've played D&D - in fact, I got into D&D as a way to 'play' Middle Earth (or something like it), so it's always been a huge influence on my thinking. Playing B/X, my Halflings in effect 'are' Hobbits - a sort of rural race of short Humans who like pie, beer and practical jokes, if they don't take too much effort. Some of them may call themselves 'Nelwyn' and refer to a hero called 'Willow'; some others might call themselves 'Hin' or 'Kender'. Players may have their own ideas about their PCs. All that doesn't matter. Halflings mostly still live in agricultural villages, elect their own magistrates (who are often called Sheriffs) and have big feasts. They are definitely like Hobbits even if not necessarily identical to them.
For Hobbits in the Shire, 'coming of age' is 33 - at least it is for males, and that's all I have to go on. Past 50 is regarded as a settled and respectable age. 111 is a grand old age, 120 is extremely noteworthy. So, more or less humans + 50% I reckon. 33 is close to 21+10.5. 50 is approximately equivalent to 34 for humans, 105 is 70+35, so 120, at 80+40, probably would be noteworthy. As a rule of thumb it seems to work OK. So for Bonifer Warren (above) his 'human' age of 23 - early manhood in other words - equates to about 36 as an actual Halfling age. If I wanted to get really granular, I'd say he's 35, and close to his 36th birthday.
Taking a similar tack with Tolkien's ages for the Dwarves: 100 is still young - Fili and Kili hadn't reached their first century (being 82 and 77 years respectively), and were the youngsters in the Quest for Erebor and regarded as 'rookies'. But more than 200 is pretty venerable; up til now I've somewhat flown by the seat of my pants and multiplied by between 3 and 4 to reach a Dwarf-age. But perhaps it should be 4-5 that is the proper range. If Fili and Kili were conceptually at least in their late teens or early twenties, and inexperienced compared to the likes of Thorin, Balin and Gloin, who were seasoned warriors, but Dain Ironfoot was very old (at 252) at the time of the War of the Ring, that implies the flourit of a Dwarf male (again, pretty much all of the information we have is about males) is approximately from the first to the second century of life. I guess this corresponds more or less to 25-50 for humans. Dain's 252 years on 4x reckoning is therefore the equivalent of 63 for a human male - when perhaps being a war-leader is not generally high on the list of men's priorities, and an old warrior could maybe have expected some sort of retirement, leaving the actual fighting to younger men. Fili on the other hand would be the equivalent of 20, and Kili 19. They might be very well regarded as young and inexperienced. If we take 5x as the base, then Fili would be the equivalent of 16, Kili 15, and Dain 50. These seem low estimates. Of course, Fili and Kili would be young and inexperienced at 15 and 16, but they're young untried warriors, not children, in the Hobbit, and if Dain is 50, that doesn't seem so old that he'd be venerable; so perhaps 4x is about right for Dwarves.
Elves are a problem for many reasons in D&D; one of the problems is age. How old are Elves, exactly? I'm not sure there is a definitive answer. Tolkien's Elves are functionally immortal, and some at least in the stories are thousands of years old. Elrond has lived for more than 6,000 years, Galadriel perhaps between 8 and 9,000 years. We don't really know how old Legolas is; he is the most active Elven character in the stories (at least, Tolkien's stories - I'm not concerned with Jackson's retellings here), the closest to a 'young warrior' and therefore - maybe! - an analogue for the PCs at the beginnings of their adventuring careers. The best and brightest minds among Tolkien scholars seem to think that Thranduil lived in Menegroth in the First Age, making him more than 6,000 years old; he made the move from Southern to Northern Mirkwood, around TA1000, and Legolas seems to have been after this, because it was after the separation of the Elves of Mirkwood and Lorien. However, Legolas was vastly older than the other members of the Fellowship, whom he called 'children' (presumably, excepting Gandalf, but not Aragorn). Aragorn is at this point 87 (approximately equivalent to 29 for the Dunedain) and Gimli is 139 (140 would be equivalent to 35 by my reckoning of 4x). So, if this is accepted, 2,000 years seems to be the maximum age for Legolas, but surely not less than many hundred years for his minimum age. He also says that he has 'watched many an oak grow from an acorn to ruinous age', which may imply he's more than 1,000. In Rohan he declares that the 500 years or so of the history of Rohan are 'but little time' to the Elves. It seems reasonable then that Legolas is around 1,500-2,000 years old.
But D&D Elves aren't necessarily Tolkien Elves. It seems that in different editions of D&D, Elves live for between 500 and perhaps 1200 years, and 'young' Elves beginning their adventuring careers may be between 25 and 125 years (not that I remember that info from any B/X-related material). B/X doesn't really deal with this as far as I've ever found. I've never come across anything to do with it in the modules or stuff from Dragon either. I do have some things from the old Pelinore setting from Imagine, which is a) official D&D and b) B/X compatible - I shall check there and see if there is anything that has bearing on this issue (I don't remember if people's ages are given in the text - if they are, the ages of any Elf NPCs might provide some useful information).
Anyway, 'young' Elves who are mortal and die after about 1200 years (I think this is how it worked in AD&D 2e, if I recall correctly) might not be how it works in my campaign. I don't have a problem with vastly old Elves, as long as it's understood that that doesn't mean that they have extra knowledge - not about anything useful at any rate. Sure they were alive on 12th Grune, 406 years ago, but that doesn't mean they attended the burial of Jarl Ragnald or know anything about what happened to his magic sword. They can however tell you exactly how the light looked through the trees (and what sort of the trees they were, in what season) as Marulan played the harp and Aldriana sang the Lay of Lost Tarathiel (and how her cloak wove patterns in the air, and who wove the cloak, and what the names were of the grandparents of the person who wove the cloak, and what was the smell of the air in the twilight as Aldriana sang, and every single word and how it referred to all the Tales before of the Age-long yearning heartbreak of the Elves...) because at the time, that was much more important.
Now some buzzing Humans want to find a stick or a bauble or something. How does that compare to 10,000 years of tragedy? Why would Elves notice what Humans, Dwarves, Orcs or whoever else was up to, most of the time? Going adventuring might be diverting, especially if involves new things, places, people, but in that case, they're deliberately trying to go beyond what they know, and probably therefore don't have special knowledge about it. Some big-picture stuff maybe ("before the Time of the Warlords, the Ebon Empire flourished - but now that people are like the leaves of a forgotten Autumn..."), but maybe not even that. As Legolas's quote about Rohan shows, even Human kingdoms are fleeting to Elves. Maybe some legends about dragons, but any detailed stuff? Maybe not. Details about a specific time or place might be details about something utterly inconsequential, from a quest point of view. How would they have known, 406 years ago, exactly what they were going to need to know in the future? Elves are pretty clever, but unless they have precognition and already have found out what they will need to know, they may not have better access than anyone else to relevant historical data. Seems like that could be how it could work to avoid the logical problem of the Elven characters being expected to know details that would help the party.
So, I'm rather minded to say that any Elves in my campaign can be as old as their players like, from about 20 (assuming growth of infant Elves at more-or-less the same rate as humans) to perhaps 2,000 (which I'm assuming is about 30, probably at the top end of Legolas's apparent age). This implies that Elves would age on something like the following basis: 1-20 - as humans; 21-2,000 - 1/66 the rate of humans; then maybe 2,000-4,000 - 1/200 the rate of humans (4,000 is about 40?); 4,000-9,000 - 1/500 the rate for humans (9,000 is approximately equivalent to 50). If players want their PCs to be older than 'early 20s or equivalent' they can use this rough guide. One of my players wanted to play a middle-aged human Fighter, so perhaps someone will decide they want a middle-aged Elf too. I probably don't need any Elves in my campaign world who are more than about 10,000 years old, so I'll regard 9,000 years (as I say, Galadriel's approximate age at the time of the War of the Ring) as the top end of Elven ages for practical purposes. But in terms of 'quest-knowledge', the point is that all that time, the Elves weren't paying attention to anything that might be important, unless it directly involved Elves, and even then, maybe they just don't know what Queen Faliria was up to. Would some random Wood-Elf from Lorien know what was going on in the White Council? Probably not; so probably some random Wood-Elf from Canolbarth Forest or some such probably wouldn't know what the Elven aristocracy in Mystara had been up to either, except in general terms.
Whether the PCs should be just 'some random Wood-Elf from Canolbarth Forest' is another matter however...