Yeah, you shouldn't keep putting it off all the time though, or you'll regret it in the end. I am 19, which is not really old, but if I had listened to myself while still in high school I'd be starting T right now and fully living as male, as for now... I am in the awkward early phase, and people ask more questions because I'm "late". Like "You wore a dress to prom in high school, why? And why tell it so late? You sure this isn't a phase?" I often wish I had expressed my feelings more often as a little kid. But whenever I said "I wanna be a boy" people laughed it off and were like "Yeah right, you can't. Go play!".
Later in high school I had so much other ->-bleeped-<- (bullying, grades, etc.) I just pushed it away and focussed on that instead. Setting it aside not to let it ruin your grades is good (no one wants an extra year in high school :s), but don't push it away too far either. You can start with talking about it with your parents, and just express your doubts and feelings a bit, then it will be easier for them to accept you want to transition later (assuming you do. It's not for everyone, so don't feel obliged to)
Good luck!