I'm surprised that no one's mentioned the problem of different races and their respective differences in longevity.
Consider this:
A human reaches adulthood at age 15; an elf at age 110. Until that time, both are more or less children.
For the next 50 years they both train and adventure, until, at age 65/160, respectively, they reach 20th level.
The human is now "Old" and 5 years shy of "Venerable". As an "Old" character he gains the following penalties: -2 to STR, CON, DEX (if he's a fighter or barbarian, it stings). He also gains the following bonuses: +1 to INT, WIS, and CHA.
The elf, by contrast, is *not even middle age*. He won't reach "Middle Age" for 10 more years!
Let's go back to the human. His maximum possible age (barring magical effects) is 70 + 2d20 years, which would, at the highest possible roll, make him 110 years old when he dies.
The elf, standing at the bedside of his dying 110-year-old human buddy, is quietly weeping. After all, he and this human have been adventuring for 95 years--since they were both young adults (15/110 respectively). The elf, however, is still only 205 years old--"Middle Age". He has a mere -1 penalty to STR, DEX, and CON, and gains +1 to INT, WIS, and CHA. Even worse, he's still *** 58 *** years away from being "Old"!
Now, who exactly is this elf's competition? Young, strapping, 1st level humans? You can start to understand why in AD&D 1e and 2e they had level limits for demihuman races. Realistically, dwarves and elves (and perhaps gnomes) should be the demigod, 60th level rulers of the world. ANY world, BTW. Pick yer setting.