Whitelisting is literally the only available technical solution. None of the others work and they can't realistically be made to. Anyone who tells you they do is either hopelessly non-technical, woefully misinformed, trying to sell you snake oil, or has an ulterior motive (surveillance).

I suppose something like the Great Firewall is also kind of sort of workable but that's government coordinated filtering so expansive that it begins to share more in common with whitelists than blacklists.

For every site that complies with age verification laws there will be uncountably many that don't. And that's before getting to file sharing networks. And then there's the dark net. Both of the latter are readily accessible outside of the Great Firewall.