I can think of two:
- Speaking multiple languages, and
- Perfect pitch
Both are more easily learned at a young age. Are there others?
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Playing a musical instrument:
Easier to learn when young and improves brain elasticity.
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Cooking:
Encourages healthier eating habits, decreases frequency on costly meals out, and makes learning to cook new things less intimidating.
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Fixing things / building things:
Once you start taking things apart or putting them together (doesn’t really matter what) it increases the confidence of being able to do your own repairs / maintenance instead of paying others to do it.
Nitpicking: I’d rephrase “playing an instrument” to “playing a first instrument”. I struggled as heck to learn the guitar as a young adult, while kids in my music class were having a much easier time; but once I got it after a while, all instruments I learned after that, even in my 40s, were a ton easier.
Swimming is much easier when learning at a young age.
Thinking like a scientist. Critical and evidence -based thinking and action can be unintuitive at times - but it’s the most realistic way to perceive reality. It needs practice, but is tremendously helpful. And the world also really needs it.
Social skills in general. If you don’t learn to trust and form relationships with people and resolve conflict early on, you struggle your entire life to emulate it poorly.
Language, other than one’s native tongue.