All kinds of laws are applied to services provided via Internet. For example, once upon a time people said collecting sales tax was an insurmountable problem and a disaster for ecommerce. Time passes and what do you know, people figured out ways to comply with laws.
Age gating is not the same thing, there is no transfer of goods. It's someone's arbitrary idea of what should and shouldn't be allowed on the internet. And it's pretty clear at this point that it's about control over information. Plenty of articles on the subject if you care to look.
You have made a claim that age gating some online services is an "arbitrary idea." I don't see how that is different from taxes at all. Taxes are likewise an "arbitrary idea." Taxes are likewise a societal control measure.
There is no need for articles to explain a very straightforward truth. If you are unable to make the case for something, claiming unspecified writings elsewhere doesn't get you any further.