Most VPN companies won't implement age verification, because their purpose is privacy. This is really an attempt to ban VPNs. This won't be popular when 70% of the population uses VPNs.
To be honest, unfortunately, I'm not really sure about this one.
I would suspect maybe <1% of the population uses VPNs. At least outside of work environments and connecting to intranets.
They will when it's law. Their purpose is most likely either snake oil, or bypassing geo restricctions on netflix or sports.