As far as being illegal or not holding bitcoin while < 18, I'm sure that the rule is mostly iffy in countries - if bitcoin isn't seen as an investment vehicle then most likely there isn't anything stated that having it under 18 years old is also illegal. We can all debate on the justification as to what's the difference between having 16-17 years old vs. 18 years, but as it stands the rules are those (and will be for the foreseeable future).
My recommendation would be - assume that you are aware of what you are actually buying and your expectations match the underlying behaviour of bitcoin (especially if you are buying it just because of what you heard vs. buying it because you understand what's behind it) - you can try to purchase voucher cards (such as Azteco[4]) and then redeem the bitcoin in those.
I'm only sharing that recommendation because I know that some P2P markets do require that you have at least 18 years old to use the service (for example PeachBitcoin[5]). Being P2P markets they won't ask you for ID checks, but I suppose that if they do found out that you've signed up under false claims they could suspend your account.
[1]https://help.coinbase.com/en/coinbase/getting-started/getting-started-with-coinbase/create-a-coinbase-account
[2]https://support.kraken.com/hc/en-us/articles/360059110151-Creating-and-verifying-your-account-on-the-Kraken-app
[3]https://www.binance.com/en/terms
[4]https://azte.co/
[5]https://peachbitcoin.com/terms-and-conditions/