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.
That's a much thornier legal issue
That's a much thornier legal issue
The first amendment is a two way street. Everyone has an inalienable right to seek and read/view any media, and the government of the United States is forbidden from taking any actions that limit the mass dissemination of media.
The whole context of the amendment is existing governments preventing people from mass printing and distributing political pamphlets. "You can write something, but not distribute it" is entirely antithetical to the point of the amendment.
The issue is not that age gates are illegal, but that the government forcing people to use age gates is illegal.
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