It took me a month to convince my parents to let me pierce my tongue when I was 16. Research it so you know all the things that can go wrong like infections. Know how to properly take care of it. Offer to pay for the piercing, jewlery, and cleaning supplies. Talk to them rationally about it without getting upset, even if that means dropping the issue for a few days. Show responsibility in other areas such as chores, homework, ECT. And lastly if they still won’t budge dont let it put a rift in your relationship with them, its not worth it. I was the youngest of several kids so my parents were a little more leniant then most.