I believe the text in the header image refers to the age limit of the playground where the photo was taken. Itch.io allows you to create an account if you are 13+ years old and, since there is nothing stated about that in the description, I assume the only requirement is that you have to be able to publish a game here. One thing to note, though, is that, according to Itch's ToS, if you are under 18, you need parental consent to publish a game.