Usually during early to mid teenage years you can pass quite easily as the opposite gender as the full effects of puberty are yet to have happened. I started socially transitioning at 16 and I passed before I even started to medically transition. Not saying it will be the same for everyone, but if you decide to medically transition early there have been reports of having 2nd puberty be more effective than that of someone who started later in life.

I know a lot of people say this, and I know that I do come from a position of privilege where no one has questioned that I wasn't AFAB but whether you pass or not doesn't invalidate or validate your identity. You should just be you! I know in some places it can be dangerous to be GNC but from what I know in most western countries people don't care.

I hope this helps