I thought it was 16 as well, but maybe it is 18, or it depends on your location in the US. I'm 33, so I don't know what the age requirements are in order to get a bank account. I know with a guardian you can start a bank account at any time. I think parents can create a bank account for their kids.
Loot boxes should automatically render a game with an M rating. Since it promotes gambling, it should be 18+, which would also require the game to be rated M due to it. I believe that's a rule that some are trying to enact. And I think that's fair. Especially if you can buy loot boxes in game. That in my view constitutes gambling and therefore should in turn result in a game being rated as such.
Loot boxes should never be in a game that's rated below Mature. But, only if it's lootboxes you get via buying in game. If you get free loot boxes from the game and playing the game, then I don't see an issue with it and therefore I see no issue rating that an E or T. But, when it sells loot boxes for real money, then yes, it should be rated M.
To answer your question, loot boxes should only be for legal adults 18+, or in some states 21+.