Firstly people will just go to the sites which have under 1/3 and therefore are not in-scope.
Secondly sites will game the threshold, if they think it'll be to their financial advantage.
Thirdly underage kids will (I would imagine) work out how to bypass these filters pretty quickly, using VPNs et al.
Fourthly, people attempting to bypass these filters might fall prey to malware dressed up as VPN/bypass software.