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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".

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