I just got on T, but it's still too early to notice changes to my voice. In general, I sound like a kid, probably 12 or younger. It's not high or pitchy, but as you say, very youthful. HOWEVER, when it comes to the legal-age-ish stuff, one of things that have worked to my advantages is the way I speak. In situations where I have to convey my maturity to the other party, I maintain a very professional and assertive tone. Kids don't speak like this. They'll question my age initially upon hearing my voice or because of my appearance, but after a few sentences, I get my way. Over the phone, they may get confused and switch to "ma'am," but I'll either tell them, "please use Deven" or "actually, sir is correct." So, if you don't think you can deepen your voice enough, then just be very articulate and direct.
(IMO, Justin Beiber sounds like a girl.)