I can think of two:
- Speaking multiple languages, and
- Perfect pitch
Both are more easily learned at a young age. Are there others?
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.