I'm in my mid-30s and just beginning to physically transition, though I've known I was trans since I was 4 or 5. Basically around the same time I realized that there was a difference between boys and girls.
For me, some of the delay was that I never saw trans men anywhere. I was familiar with trans women and the idea of a "sex change" while growing up, but never fully realized it was something I could do.
Once I had figured it out, I was just old enough to be worrying about getting and keeping a job and tried to put all that gender stuff on the back burner. Transitioning is was expensive and strange and could cost me my job or my life, right? No sense in transitioning when my partner was willing to accept me as a man behind closed doors, right?
Wrong. As a lot of people on this board have said, eventually you reach a point where something has to give. I know I can't be happy living the next 40 or 50 years as a "woman". And I refuse to be unhappy for that damn long.
PS: Jasper is an awesome name, btw. I'm glad you're exploring all this so early in your life; it always cheers me up to see younger folks doing that.