Interesting idea, but what would happen if someone got killed? Would they respawn as a baby? There could be some noob that kills everyone, and then people never grow up.
But if you could somehow make it so you can't die over and over, the idea would be great for RPG's.
It would also be nice if the server admin could choose what age everyone starts out at.
You could also do this with events.
Onrespawn-->player-->setsize-->n n n
Then, set a long delay, and make size bigger.