Seconded. If the PrC should be level-based, then its prerequisites should include something level-dependent like BAB, or skill ranks (even better). Only rarely are the feat prerequisites for a PrC so high (like Whirlwind Attack) that a human fighter couldn't achieve them by 2nd or 4th level, while simultaneously excluding all level-dependent prerequisites. Usually the skill ranks will limit the level anyway, since max ranks is set in stone.
(Come to think of it, I'm marginally suprised that I've never seen a homebrew class/feat/PrC/etc that tried to increase max ranks above level-normal. Hm, that's quite an idea.)