I'm in your exact situation. Just that I am 17. Even as a female everyone tends to think I'm 12 or so.
I'm not out neither, but well, I kind of pass (although my parents usually screw it) so I have no problems at all when I go to the men's section (well, youth section). I recently got a button up that's pretty good. The only thing that I must say is that when I asked for the fitting room the woman treated me like a ten years old, she even asked for my family. xD The good thing is that she leaded me to the men fitting room. I was so happy. xD
Don't be scared of the youth section, there are good things there aand they're usually cheaper, so it's a good point. It's better to use something that fits you rather than something from the men section that will make you look like a kid wearing his father's clothes (or worse, like a girlfriend wearing his boyfriend's clothes).
For the passing thing, well... Just act your age, maybe be a little bit serious. Talk properly, that helps (although I get a lot of anxiety when socializating so my voice tends to get higher and I use to slutter and that doesn't help me). The hard thing is passing as the age, at least in my case. It doesn't bothers me that much 'cause I'm used to being "adorable", but that's just my case. Try to keep your voice low and don't look like if you were looking for your mother or something. xD Be confident about yourself. And I, personally, would recomend to use dark colors in general, I don't know, I find them better for passing and they're not childish.
Happy holidays and happy new year!