This is standard practice in the UK too - if you're buying alcohol as a group, they'll ask for everyone's ID if any of you look too young.

But this isn't specifically written into the law, i.e. the law doesn't say "you must check the ID of everyone in the group." Rather, it's that businesses receive ENORMOUS fines if they're caught selling alcohol to underage people - so most of them have very strict internal policies to minimise the chances of a false negative.