They (pornhub) do ask people about their age, depending on the country they're visiting from. For Russians they even tied confirmation process to one of local social networks (source in Russian): https://tjournal.ru/flood/46247-kak-rabotaet-proverka-vozras...
Funnily enough, I think it's fair for pornhub to assume their audience is 13 or older.
Porn sites used to make you affirm that you were at least 18, though. I wonder what happened to that.