Why are actors short?

Most of them got their start as children or teenagers playing parts two or three years younger than their actual age. Older children have longer attention spans, and older teenagers have more acting experience, so the ones who are small for their age have a pretty strong advantage. There also seems to be a strong tendance to avoid having teenagers appear taller (or even as tall) as their parents. Even an eldest son is almost always played by an actor who's decidedly shorter than the actor playing the dad. This seems to be a fairly strict rule in casting family sitcoms, and almost as predictable in films. It's almost laughable to see it played out so consistently even in recruiting commercials for the military. It's just really hard for the taller actor to get the same kind of early start in younger parts.