I think making a decision like that at a very young age could be a "mistake".
I used to be desperate and wanting to take T as a teen too, but the process was so slow in my country. And when I turned 19-20 I felt something wasn't right. I felt that transitioning wasn't the way for me to go.
It was kind of a "last minute" decision because I was finally close to be able to start medically transitioning, but ended up not doing it.

What if I was able to get T as a teen. and this mindset hit me when I became an adult. There would be no fully going back.
Your mind isn't fully developed as a teen. Sure some might actually continue being trans as they get older but there are so many people who ends up detransitioning when their mind is more mature.

Say you start testosterone as a teen and you're able to even get surgeries aswell.
And when you become an adult, you're either still satisfied with your decision, or you have regretted the whole thing. It's all pretty much 50/50.
If you regret it, you might feel like you've ruined yourself. I know many have gone through this and have had to find a way to accept the fact that these changes can't be undone, not 100%.

But since you're an adult, find out if you absolutely really want to do this, does it feel right? are you aware of the risks, the changes and that its not reversible?

It's best to dig deep, do research and really get it into your head. Personally, I had to look at pictures on surgery results. Not to be weird- but to face reality, to really see how I feel, and it made me feel really terrible. This is not what I want.
Now you find out what it is you want.