To be honest I'm not completely sure - the option for mature content states "not suitble for minors" and the full game will deal with some quite dark content. Perhaps my understanding of the rating and how this game compares is actually not quite accurate. I'll look into it!
Itch has no actual age rating. Their effective rating is 13+ and 18+. You need to be 13 to use itch according to their tos, and they have a section that you can activate, that is in all intents and purposes for adults. But is vaguely named non safe for work. Or not suitable for minors.
Age ratings for games and movies are different around the world, and how they are enforced. Itch is US based, and the US ratings for video games are E (0), E10, T (13), M (17), A (18), with several additions. Also different around the world is, what would require a higher rating. Violence in general will give higher ratings in Europe and Sexual content will give higher ratings in US and vice versa.
But your game is not officially rated anyways. Maybe it will be for your Steam presence.
In practial terms, unless it is porn or graphic ultra violence, it will be a rating of less than 18, and itch does not care if it is 13 or 17.
From your comments I gather, you would rate it 17.
Steam might give you this warning message, if they also deem it 17.
THIS GAME MAY CONTAIN CONTENT NOT APPROPRIATE FOR ALL AGES,
OR MAY NOT BE APPROPRIATE FOR VIEWING AT WORK.