But probably memory as well. If I start specializing elsewhere I'll forget the intricate details of the thing I'm already specialized in.
Rather, it's that you have only so much time to dedicate to things... if you're focusing very much on one, then you're not spending time with the other -- and overtime, you'll lose that other thing. If you can manage to spend a good amount of time on two things (let's say, learning a new instrument and learning a language) everyday, consistently, then you will get both.
You can create ungrounded abstractions all you you want, but as an older person who has commiserated with other older people, the common conception that it's harder to learn and retain things in older age is a real thing
- Homer Simpson