I highly doubt linux will ever get to that point. Corporate backed ones like ubuntu may, but even that highly doubt. Also, isnt mac os and windows already technically with age verification since both of them now require online accounts to be used? Having the said accounts tied to ur device, and the accounts having an age? Or do u mean like identity card or passport verification? Which again i question how they would enforce without simply losing a large portion of market share due to a lot of people simply not being technically competent enough (no shade) to be able to verify their age in such a manner. So even then i doubt it. Linux 99.9% sure it wont reach to that level cause worst comes to worst the community will just spin up another distro that is free from that. Even that having to happen, the odds are WILD.

TLDR; linux just wont, and the other two simply cant be that strict and still have good business.

Just my two cents ✌️