I'm afraid that's a common misconception. Middle aged people learn new things all the time, in my experience. It's a matter of choice, willingness, motivation or whatever you want to call it.
I worked as part of the Bonn Gerontologic Longitudinal Study when I was doing my Psychology degree at Bonn University. This study did a lot of mythbusting with regard to the normal notions regarding ageing.
One of the biggest myths to be disposed of was the until then accepted notion that intellectual ability automatically declined with age. It was a study that went over 20 years.
With regard to computers I have pretty much lived with the fact over the past 30 years that I regularly have to dump what I know and relearn. Although with some things it is very much a case of, "The more things change the more they stay the same".
I type on my completely blank "DasKeyboard Ultimate", it only has the raised notches on it for the "F" and the "J" keys. So when I am writing it is basically as if I were talking to you rather than actually going through a writing process.
You probably know some people with Asperger's but don't realise it, just as with gay people. I know that I remained in my own closet until about six years ago because of the stigma attached to autism.
When I came out and revealed the fact that I was autistic it went about the way that I thought it would with some people shying away and others saying they weren't bothered (but until this day still don't react to me the way they did) and a few who really aren't bothered by it.
Cats, dogs and computers tend to love me, humans not so much.

Three out of four isn't bad though.