You're thinking of vampires.

[It still gets a small laugh out of him as he shakes his head.] You're not the first one, kiddo. Doubt you're going to be the last.

Seriously, though. This is the sort of thing to wait for until after you've finished growing. Shifting involves all of your bones breaking and reforming when you go either way. That takes a toll. I'm not kidding when I say it'll stunt your growth.