Actually, shônen 少年 (omit the long O at your own peril) literally means "few years", and the best translations would be "kids" or "juvenile", as in shônen rôdô, child labor, or shônen hanzai, juvenile delinquency. Generally speaking, shônen refers to children who have not yet reached puberty, ie. pre-teens; after 13 or so they graduate to being yangu, seinen or wakamono, depending on whether you want to borrow your word for "teenager" from English, Chinese or yamatokotoba.

However, while shojo means specifically young girls, there is no corresponding term for young boys only, which is why shônen is sometimes used to refer to boys only (eg. shônenshôjo, "little boys and girls").