No source but a possible counter-example: SIM card carries the date-of-birth of the user (because, really, part of the problem is that you cannot supervise what a kid does with their phone 24/7), age-restricted websites have a flag in their HTTP headers, browser get age from SIM card through the OS. Same goes with app stores etc.

Like copy-protection there's always a way to circumvent it, the name of the game is to make it difficult enough. And for parents, to make it very easy to set-up.