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").