They do [0]. You can also do it in their app if you are on a contract and therefore over 18 by definition.

[0] http://ask3.three.co.uk/srvs/cgi-bin/webisapi.dll/,/?new,kb=...

In my experience that never worked. I eventually gave up trying to use them.

You can do through their app. But it is almost always broken. It took me months before I managed to get the app not to give me a 'temporary error'.

How would having a credit card prove somebody‘s age?

You have to be 18 to get a credit card

That's definitely not the case everywhere. I've had a credit card before I was 18.

It doesn't have to be perfect - the blocking thing is only a regulatory compliance and PR thing about how they're "protecting" children. As long as the regulators are happy and the PR is working, nobody internally cares or wants to make it 100% bulletproof (also because making it so will increase the support burden helping users who can't use the bulletproof verification flow despite being over 18).