It's back to the same question from the early days of the Internet. "How do you know the person you are talking to isn't a cat?". How can you know they aren't a cat or maybe a clever monkey taking a break from typing Shakespeare, or how old they are, or who they even are?
You can't.
You can implement every security measure available and anyone can simply lie or bypass it.
If by some miracle they actually implemented some age verification tech for OS and websites and the Internet as a whole, it will just pop up an underground market of "fake IDs" or hacks to get past it.