Do developers have the obligation to indicate for what age the content they upload to the platform is suitable?
Yes, individual platforms do not have the manpower to play through all video games to decide what content rating to give for a game. So in most cases, it’s the responsibility of the developer to be accurate about the content of their game.
On the part of itch.io is there any control over this issue?
While editing your game, you can mark it as 18+, and Itch.io takes care of letting the user know this is an adult game.
Hope that helps