There's a big difference here, in the US anyways, neither alcohol nor nicotine have first amendment protections. Basically all content delivered over the US does.
But isn't putting something behind an age gate similar in concept to putting it behind a paywall? The speech is still there, whatever it may be, just has conditions for access.
But there are already laws that, for example, restrict children from buying pornographic magazines. These have been found by the Supreme Court to be constitutionally compatible. I don't see why this would be different with similar laws that apply to online services.
Generally speaking, things that you sell (the legal term is commercial speech, iirc) is more able to be regulated by the government.
The government can ban the sale of those things to minors, generally. So the category of porn sites that require a credit card and pay gate the content might be regulateable.
But that's not how places like pornhub or xvideos operate