I guess I'm more interested in whether my ability to learn machine learning (or whatever the equivalent is in 10 years) will decrease over whether I'll be able to remember a 7 digit number.
Call that what you will.
I am confused about what you consider "ability to perform". Education is a context for performance.
The theory is that your ability to memorize a 7 digit number (working memory) IS fluid intelligence.
The problem is that memorizing an x digit number is proven to be trainable, without much carry over to g or even other working memory tasks. I.e. - it's hard to train fluid intelligence rather than train the specific task which it is applied to.
Does anyone work for Udacity/EdX/Coursera? I would be interested to know if/how performance on those courses decline/improve with age.
It would also be interesting to see if performance is correlated because of lurking third factor. (My money is on exercise, or time).