In a nutshell, the BA Galleries lounge minimum age of 21 because of the unmanned bar



(haven't had such an issue with other US lounges with bars)
However, I just noticed that if you check in online and print your boarding pass at home or send it to your mobile device, you'll still receive the lounge invitation. Only if you check in in person and request boarding passes on airline stock, you'll be denied an invitation and receive a $25 USD food voucher.
Even after checking your bags, the lounge invitations are still accepted by the lounge attendants.