All mobile network connections already come with content filters enabled in the UK, adult or not, and has to be explicitly disabled.
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.
When purchasing an internet-enabled device the UK could regulate that large retailers must ask if the device is to be used by an under 18 year old. If they say yes, then they could ship with filters enabled. They could also regulate that all internet-enabled devices which could be sold to children should support child filters.
If we did this then whether or not a child views NSFW material it will be on the parent, instead of the current situation where whether a child can view NSFW material online depends on the age verification techniques of Chinese companies like TikTok or American companies like 4chan.