Legally buy? Bitcoin doesn't have age limits. What you mean is you can't open an account with a CEX. Good news, you don't need one.

What you want to do is use a Bitcoin DEX. You sound smart enough to learn these. I use Robosats which is a Lightning Network based DEX. You can buy sats there and swap into an L1 wallet you want. Or learn to use Bisq. HodlHodl is maybe the easiest. Here's a site with a list of the main bitcoin DEX's and ranked:

kycnot.me

These DEX's have a bunch of different payment options for their offers. CashApp is one (If you live in the US you should be able to open one at age 16). Zelle with your bank account is another. F2F (face to face in person). Gift cards. ACH. Wires. PayPal. There's a bunch of options. Just watch some tutorials on whichever platform you use.