Megalomaniak In the UK, it's a crime to "supply" a minor with a "mature video presentation" (games included), unless it's "exempted supply".
I'm not a lawyer, nor british, but I'm pretty sure that means the only "crime" in underage gaming is whoever sells/gives the kid Battlefield 3 while holding some legal status that involves giving stuff away or selling stuff. Stores, government, charities.
it's under section 11
https://www.legislation.gov.uk/ukpga/1984/39/section/11
Basically, it's not a crime to buy your 15 year old Halo Infinite for christmas. Not that it matters since no one actually enforces the law
In America there's no law at all as far as I know, it's purely a "this game's kinda violent" message to the parent. In fact, my first COD game was for the Wii, and I got it because my mom asked the dude at Gamestop what to get a 13 year old.
Imagine if you had to send Steam your state ID to prove you can legally buy the big boy games.