On the post detailing PC access, some important points should be edited/added:

1. The person holding the credentials to enter MUST be at least 21 (as in the PC member, OP Gold... so someone who otherwise holds credentials to enter but is under 21 cannot simply find a random person over 21 and guest them in.
2. There is exactly ONE exception to this rule, and that applies to anyone under 21, flying alone, on BF. In such a case, the passenger DOES have access to the President's Club. However, upon entering, the person at the check-in desk will escort the passenger to the bar and introduce them to the bartender so that the bartender know he/she is under 21 and not allowed to drink. (This applies only to BF. Domestic First, International First, International Business, Latin First/Business... do not apply, and anyone under 21 flying those routes will be barred entry from the PC.)

However, I do not right now know all the rules concerning access to contract lounges, affiliate lounges, *A lounges, etc. in the USA. At international locations, where (most countries have a) drinking age of 18 and not 21, anyone with credentials over the age of 18 should have unrestricted access.