A family game has to work for a nine-year-old and an adult at the same table without either of them being bored. That is a much harder design problem than difficulty settings, and the games that solve it usually do it by making the skill gap not matter.
Whether the person losing is still enjoying themselves. The best family games are built so that being behind is survivable and being ahead is precarious.
Local multiplayer, quick rounds, and a way back in for whoever is behind.
Games with no punishing failure state, where a child can make progress alone.
Co-operative rather than competitive, which is usually the better call with mixed ages.
Mario Kart 8 Deluxe with smart steering on, or Super Mario Bros Wonder — both let a child and an adult play the same thing at very different skill levels.
It Takes Two and A Way Out are both built entirely for two people, and Portal 2's co-op campaign is a separate game from its single-player.
Age ratings cover content but not difficulty or frustration. The reliable check is watching five minutes of unedited play footage before buying.
Start from something specific rather than from the genre:
Or by the machine you own:
Or start from a game you already love — every title we cover has a page of hand-written recommendations, and the best-games lists cut the same library by era, audience and mood.