Umm.. Maybe he burned off all his hair roots so he can't grow any anymore.. =)
(When he learned to "surf") Also, how old is he? It's never told.. Maybe he's just 13-17... =)
You never know.. =)
That slip-up happens in most Tarzan movies (if not all) as a way of making him appear more attractive. Disney actually made him look more savage than in other movies by giving him long hair. These could be a spoof on older movies (like the whole "me Tarzan you Jane" thing).
Pikaew, adults don't have beards because they shave. I don't think Tarzan can do that if he doesn't have a razor and doesn't know how because he was never taught. So I do think it is sort of a slip up, but it IS a cartoon.