Our general rule of thumb was ok up to about 10 or 11 years old, in public. Beyond that, long pants was the standard. In a men only situation (like a gym, pool or camp) then shorts for adults were still ok.
Oddly enough, camps (the black-hat ones, I mean)seem to turn a blind eye towards older boys in shorts, even though the grounds are packed with girls and women. Kind of a double standard, as they would never allow girl campers to dress down if men were present.
Go figure.