In the UK, I don't think BA really care (or that the dragons can tell when they scan your BP). SFO don't give a toot, either.

The only time being U18 was an issue is when some overzealous UK Border people decide I can only be released (let into) the country through the custody of an adult in arrivals. One particularly entrepreneurial border person once decided to take it upon himself to give me a London Underground pop quiz when I told him I'd make it home fine myself, thanks.