There is no legal age to buy bitcoin if you are sufficient to appreciate it, you should be enough to buy it. However, according to the KYC requirements, most exchanges apply the minimum age of 18 years. It does not matter how much you have scanned your documents and signed your name: If you are a minor, then you will not enter it, then how can miners purchase breadcrine without breaking the law?
I believe if there is a restriction on underage. Because they might be a victim of scammers.