And now a simple-minded solution to the problem of level limits for demi-humans:
The basic deal here is that demi-humans may continue to advance past the traditional limits, but their XP charts don't flatten out the way they do with humans. The numbers above are good for Labyrinth Lord, but similar charts could be easily whipped up for any version of the game where race equals class or where the class list is short.
AD&D Monster Manual part 57
52 minutes ago