Is it normal for teenage boys to shave their legs?

It can be. It is not at all uncommon or strange for male athletics to shave their legs or, in the case of swimmers, even more extensive regions of their bodies.
Fashionable. A lot of kids now shave everything from the neck down. For the past couple of years, the unshaved teens have been much less common in my practice. They all want to be different......Just like everyone else.