Any many websites certainly do have effective measures against VPN users. Often pages won't load, or you get hit with multiple captchas.
To be actually effective though that requires that the website know the CIDR of every VPN provider on the planet, which I doubt they do. Sure they can block the big ones that advertise on Youtube and podcasts, but there are plenty of others that are most likely flying under the radar. I'm running an OpenVPN endpoint, for example. They probably don't have my IP on their list. There's no magic "I'm coming from a VPN endpoint flag in TCP" that a VPN provider can set, and no privacy-respecting provider would if there was.
And why should one jurisdiction get to destroy the privacy of the rest of the internet so that their age restriction laws get to be enforced? Blocking VPNs is only necessary if you need to geolocate someone so you know if you need to check their ID. Take away the ID check and there is no need to block VPN as it buys you nothing in terms of blocking minors.
The only really "effective" measure that respects privacy is to label content, and then parents need to parent and, if necessary, only provide children with devices that are locked down and block inappropriate content. Or don't. There are multiple ways to raise a child.