One of the 3.5e Dragon Magazines had Chromatic Dragon monster classes that took you from Wyrmling to Young Adult, iirc.

Prismatic Dragons are ridiculously powerful though. Then you also have to factor in that this is Epic, and epic levels are ridiculous in and of themselves as far as monsters, challenges and opportunities go. Looking at the Wyrmling though, it seems somewhat doable. It only has a caster level of 5, and some of the abilities aren't that amazing. It's mostly the ridiculous ability score modifiers that would break it as a PC (+18, +12, +10, +10, +10), and a monster class would be able to hand those out apart enough to make them tolerable, I think. Given that this is epic, I'd probably stick an LA +7 in there, and make a 30 level monster class with an LA-level every 4 levels or so. It's probably still horribly broken, but it is an even number, which I like.