When you buy wifi, they already make sure you're an adult. They ask for proof of residence, you sign a contract. Children cannot buy wifi. Go ahead and try - no ISP is going to write a contract with a child.
Wifi, like tobacco and alcohol, is already age restricted.
The problem is the adults buying it then turn around and just... Hand it to children. That's not the fault of the law or society.
Like, okay the store clerk might make sure when I buy a pack of menthols I'm of age. But if I just go home and hand my kid the pack of menthols, all bets are off. That's not the store clerks problem, he can't and won't get in trouble for that.
Parents and establishments are being stupid here. Same applies for public wifi. Don't want kids to use it? Okay, give it a password, only tell the password to adults. Easy peasy.
The law can't stop parents from being stupid.