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German board games have a self-rated "suggested age" that is more about the cognitive difficulty and the themes, so a non-violent game about politics would be 12+, Chutes & Ladders is 4+ and Catan is 10+.  Probably not a lot of four-year-olds on itch though...

But in that context, "adult themes" would be more along the lines of "Kirby does his taxes".


Or "Yoshi cooks his ledger file", if you want to build on that.