Okay, this is absolutely brilliant.
I'd like to say "I told you so", but unfortunately, I didn't tell you - and didn't actually think they'd do it, which makes it rather difficult to pull of the telling you so effectively.
So, never mind about that. I'll go back to my original idea: this is absolutely brilliant.
It manages to pull off a number of rather tricky feats, the main one in allowing players to play monsters when other characters are at the first level of experience.
Another interesting feature is that it allows a player to play an immature monster. Although not explicitly stated, the DM could (if he wished) restrict advancement in the monster class according to the age of the character - thus, an adolescent minotaur could only gain the last few levels when his or her character reached the required age.
I'll say it again: this is absolutely brilliant; what I tell you three times is true!
Cheers!