I forst joined BAEC when I was 16. I created an account for my dad then opened an household account just to have me joined in and collect points.

When you are 18 you can use your dad's account and kick yourself out via his login.

Regarding to your age -- BA may well check your DoB against your passport one day, which may bring you to unknown troubles.

As to lounge access -- no one said a word about it. I managed to enter ALL T5 lounges without an issue when I first became silver at 17 by myself.