most homes don't run their computers in multi-user mode. Even when they do most kids will learn the admin password because parents don't select good passwords.
Kids are not stupid. Even if some are, there are many kids in school and so any bypass will spread kid to kid fast. For each of the following reread this paraghraph to remind yourself all kids will know this.
kids can install linux /bsd on a raspberry pi or old/cheap computer - this is something I want to encourage. that of course means they are root and can claim whatever age. They can likewise do it in a vm.
if there is any security flaw kids can use it to change their age.
many programs will not check when they should. Kids will install/use these instead. this is a likely exploit vector of actors (in foriegn counties) that target kids - release a new program that does what the kids wants while also doing what they want. (Websites have done bitcoin in javascript while you read them)
who will check? onlyfans probably will, but small web sites spring up all the time, and they won't bother - many are already illegal (either copyright or illegal content).
Meanwhile many programs that we want kids to use won't bother to check. why would things autocad check - they target professionals but kids can use them and may even have to.
the above is not a complete list!
Age is useful for 'buy cigerettes' that is illegal for kids in some way.
However most of what parents care about isn't automatically bad and I know plenty of panents who are frusterated because we can't controll things how we need to without being a helicopter parent (bad). Playing video games is fine in moderation - AFTER YOUR HOMEWORK IS DONE - but we don't get an easy way to enforce that. My teens are old enough to stay home alone and do homework - but they will not do their homework when they can do something else (this problem has been around since school)
my kids phones have parental controls that I turn on. However they lack a way to enforce homework vs play vs sleep time. There is likely more, this is just what frusterated me yesterday. some things are not gated - my kids have got up at 3am, and connected their school device (under school control not me) to their phone hotspot (turned on who knows when - I can't block that at all) to play a game that the school will block next week when they figure out it is one kids are playing but they shouldn't.
people are proposing age verification because they have no idea what else they can do and are frusterated at how bad things are.