This might be controversial, but for me personally, I would tell myself to not be so afraid to try HRT, that I’m going to take it later anyway, and that I would regret not starting earlier.
I ended up getting HRT off the Internet at 21, but I knew about DIY HRT, and was tempted to buy it, back when I was around 16 years old.
Mostly I was nervous about people noticing that I’m trans. I really thought that I would look like a super masculine guy but with big breasts. Eventually I got to a point where I didn’t care what I looked like, I just needed to try transitioning. Luckily things turned out well for me in terms of appearance, but you know, it could’ve been better if I started when I first knew I was trans.
At the time, my mental health was pretty poor, so I didn’t really care about side effects (fortunately I’m doing a lot better now).