Assume for this discussion that the players involved prefer for there to be a measure of balance between the PCs regardless of race or age.
How do you reconcile the differences between short-lived and long-lived races in your game? By this, I mean how do you "balance" between a 300 year old elf and an 18 year old human in terms of their abilities?
For my games, this was always a combo of fluff and the rules/numbers.
As I figured it, the long-living races were simply slow maturing (as the age-category tables in the books should). The 18 y.o. human and the 120 y.o. elf were the same...mentality/maturity wise...in fact a hard-working 18 y.o. farm boy was probably significantly MORE mature than an "18 y.o."(120y.o.) elf.
What had the elf been doing all those years, you ask? How many words for "leaf" and types of trees do you think elves have? The names and types of the animals of the woods, every bough and branch that is suitable for traversing or construction. There's the decades of play and tutelage to hone their senses (to explain such things as the secret door detection and/or not getting "surprised", for example) and learn about fun and dance and...frolic...there are the years spent in games (and maybe more formal training for the more "serious" elves) in archery and sword play (to account for the racial "automatic" +1 with bows and swords in early editions of the game). Elves weren't (solely) "genetically predisposed" to being good with bows hence the bonus...it was a crunchy reflection of their fluffy long lives.
Also, at least in Basic D&D, "Elf" characters advanced in level the slowest... needing double (or almost) that of all other classes. When 1e AD&D came about, elf characters (and half-elves and often all other demi-humans I saw played, for that matter) were almost always kept in this "multiclassed" vein...needing massive amounts more XP to increase in level.
So, while the human fighter or thief is leap-frogging through levels, the elf fighter/mage is still at striving to become a 3rd level MU.
There is also the flavory bit from Basic & AD&D of racial level limits. Now, for my own games, we never used them. But I see the point of it for attempting a "balance" of races...the long-lived races have the time and opportunity to become very skilled in many different things...but they're ability to do that also means they have limits to how far that skill can go.
Whereas the short-lived (which was basically just humans, maybe halflings if you want to count them) were not so limited...in most instances. Their "souls" were limitless or somesuch...
Surely the elf is going to be much more highly skilled (even if most game systems seem to ignore this), particularly if they left their Elvish family when very young?
Well, they are. That's just cultural color/fluff. It's always been an accepted "trope" in games I've been in, that elves
are just better. That does not translate into the idea of very young or very old elves trying to go a'venturing. It simply, culturally, isn't done.
They are certainly more skilled. They are also culturally more aloof. Maybe a 1,000 y.o. elf IS a sorcer supreme of the Prime plane...but it is VERY unlikely, unless the PCs get to very high levels themselves, that elf is never going to be encountered.
They are more "removed" from the rest of the world. A "very young" elf isn't leaving their family early. I had made it a general rule in my game that any elf encountered outside the elvin realms would have to be at least 150 y.o...and that was essential "early 20's" when an elf might desire to explore beyond their home-wood....they know all of the words and how to read the trees and wind, they've got their detect secret door thing down and are confident in their general skill (which is above/beyond most other races) with their bow. So...time to go see about the rest of this world.
And what happens if you compare that 18 year old human with a 35 year old human who has seen almost double the years? How do you "balance" them at the game table?
"Balancing" different ages within the same race isn't really...possible, or necessary, I don't think. That's really a matter of character back story.
Is the 18 y.o. a noble's son who's had tutors in swordplay since he was 8? Is he a farmer who married young and lifted his first weapon in combat when goblins destroyed his homestead? Is the 35 y.o. as someone mentioned, been a farm all of his life until said goblin raid? Had he been sequestered in a monastery for the past two decades and only just completed his training and "took his vows" to become an "honest-to-goodness properly-ordained spell-casting cleric."
The 18 y.o. nobles son and the 35 y.o. newly ordained priest and the 150 y.o. mu/thief are all 1st level characters. There's their balance. The 35 y.o. priest might advance faster with some Wis. bonus to their acquired XP. The 18 y.o. will advance quickly without any "aged experience" because...well, fighters advance quickly...chalk it up to "youthful exuberance"/eagerness I suppose. The elf, yes, chronologically "older" is no more "experienced adventuring in the human world" than either of the other two...and his multi-class will ensure he advances much more slowly than either human character...
Thus generating, if I'm understanding properly, the oft sought "balance" between characters.
You can follow the vague "age group" ability modifiers. But, I think on the whole, the age of the character and their maturity and their acquired skills are really more dependent on their story (as interpreted by the crunchy bits on their character sheet).
Seems to me though, if you are going to allow those being applied, or, if you want Age to play a role in the character balance in general, then you have to kind of follow the established "Starting Ages" for various classes..which I think it safe to say were almost universally ignored by groups I played with. hahaha.
um...I dunno if any of this helps...I seem to have flow myself down the stream of consciousness. But I
think my point was, in the "older/pre-3 editions" reconciliating long-lived v. short-lived races was a combination of "in the game world" accepted hand-waving and assumptions about the culture that played into/explained some of the rules, "special abilities" and bonuses allowed the long-lived races. So between the two we never really thought of varying ages as "unbalanced" or not making sense in the game world/narrative.
--Steel Dragons