Older editions of D&D limit what classes non-humans can take and limit how far they can progress in those classes. The idea was that if you didn’t have these in place, demi-human characters would quickly overwhelm human ones, both because they have advantages at first level and because significantly longer lifespans meant that they should be able to continue advancing well after their human contemporaries have died, causing humans of significant level to be completely outnumbered over time. Of course, there were still players interested in demi-human characters and as far as I can tell these level limits were never that popular. So at first glance we’re left with a dilemma: retain level limits and the dissatisfaction they cause, or remove them completely and subordinate humans to other player character races.

Now, there’s no shortage of other options – perhaps the best-known is 3rd edition’s decision to give humans bonuses at first level to make them more competitive with demi-humans. That still didn’t solve the problem of elves continuing to level up at age 500, though. I think we can solve all that easily enough, though (and as a side-benefit the rules are short). Note that I came up with these with OD&D in mind, but I don’t see any reason they wouldn’t work with editions as recent as 2E – basically, anything with a notion of prime requisites (though in editions where race = class you’ll have to work up a progression beyond the listed levels, or let non-human characters switch classes on that – maybe I can get to that later).

The central idea here is that characters can be of any class regardless of race, and there’s no preset level limit for a character based on race; advancement is only limited by their ability to continue gaining XP.

1. Typical demi-humans have a 10% XP penalty (races with greater abilities than the classic dwarf/elf/halfling trio may have a larger penalty).
2. Any class outside those “allowed” (i.e. typical) for the character’s race imposes an additional 10% penalty to XP applied toward that class.
3. Once a character of any race has lived longer than a human can (100 years, say) they face an additional 80% XP penalty.
4. XP modifiers for high or low prime requisites apply as usual.
5. Multiple XP modifier percentages can apply at the same time, and are added up rather than multiplied. So an Elf over 100 years old faces an XP penalty of 10% (elf) + 80% (outside human lifespan) = 90% in general; for classes other than Fighter or Magic-User (only allowed Elf classes in OD&D) it will be 100% before adjusting for prime requisite scores.

The consequences of this are that demi-humans may still be slightly superior to humans in their earliest adventures (when simple class differences like Fighter vs. Magic-User means that balance isn’t too precise in the first place, and at any rate low HP totals place everyone pretty close to an early grave) and pay for their advantages over time with slightly slowed advancement. Characters that want to continue advancing after their human colleagues have aged out will best do so by sticking to the career paths archetypal for their race and those for which they have any particular talent (XP bonus due to high prime requisite scores); even then their advancement will be sufficiently slowed that the world shouldn’t be overrun by super-elves or what have you.