Any suggestions on how to restrict ages under 14 in registration?

I am having a tough time finding and coming up with ideas on how to restrict young people who are 13 or below to register or not be able to play our game. It will be a big help to hear your answers. Thank you!

What platform are you making your game for? This isn’t something that is done in unity but is something that is done when you upload your game to steam/ android services etc, before you publish it.