Why do you think shops ask for proof of age when you buy cigarettes? Not because they care about cancer or want to sell less, it's because they're required to by law. Of course, teenagers can still find workarounds. They can ask an older friend to buy it for them, just like they can use a VPN to access porn.
The difference is, regulation shifts accountability. It moves the responsibility from a greedy, insensitive business owner to the kids. And at least with the kids we can guide them, and help them spend their time and money where it actually matters.
Note: I know people who love guns or porn are probably going to downvote this, but someone has to say it.
This isn't about protecting people.