In the US, with parent permission, you could probably get a doctor to prescribe it at 16. But expect to jump through some hoops. You'd most likely be looking at 18.
You would either need to: (1) go to therapy, get a referral from your therapist for HRT, find an HRT provider, get a prescription from them; or (2) find an HRT provider that operates on the informed consent model, and get a prescription from them.
The second way is obviously faster, but if you're planning to have insurance pay for anything later on in your transition, you'll want to make sure that you're following their requirements. Some want you to do therapy, then T, etc. before they'll cover any kind of surgeries.