At a guess, I would think the majority of people on FB will be under 18
fb is more used by older age groups, biggest group are the mid-twenties to mid-thirties, kids are more into tiktok these days (never used either myself)
the trouble with any effective identity/age verification platform is that
when (not if,
when) it gets compromised, the potential for exposing sensitive personal data is enormous - banking/card info, passport/ni/licence/other government id, age, name etc.
the law is just another example of how badly politicians get things wrong because they "must do something", and waste tax payers' money chasing headlines rather than attending to serious issues
to have an isp account requires more than enough to verify age, as does a mobile phone contract, in both cases it needs adult-level resources such as a bank account, utility bills etc.
the fact is that it should be the purchasing adults' responsibility to ensure their kids don't misuse the service that the adults control, there are plenty of existing options for this, isps offer default blocking
payg data can be bought for cash, but the isp knows it's payg, simply default-block payg access to 'adult' sites, the isp can do it easily