It probably depends on the context.
Technically in GLBT gay = male homosexual (but generally gay is also applied to females pretty commonly too), lesbian = female homosexual, bisexual = male or female attracted to both sexes, trans = (something like) person whose physical sex does not match their gender identity. So technically no--the literal meaning of "gay youth" wouldn't include bisexuals since bisexuals aren't homosexuals (attracted to the same sex), they're bisexual (attracted to both sexes). It wouldn't include trans people either, since "gay" is a matter of sexual orientation while "trans" is a matter gender identity. So a trans person could be gay, lesbian, or bisexual (or queer, pansexual, whatever) in addition to be trans.
But I think gay is starting to be used as more of an umbrella term that refers generally to the glb community. (I think it's less likely to be used to refer to trans people.) People talk about "gay marriage" which is technically a misnomer, but it's the easiest way to refer to the concept; and surely anybody who is against "gay marriage" isn't splitting hairs about who's bi and who's gay--they're just against all of us! I am bisexual but once in awhile I will refer to my "gayness" and I have some friends who refer to hanging out with "the gays" while spending time in our group of friends of mostly lesbians and bisexuals--but that's all in a joking kind of way.
So I think the answer is yes and no.
