Desk sitting and fussing at the computer is no good for seniors or for anybody else for that matter.
You need to be building muscles and brain cells, which you can do at any age.
So, when a problem comes up, use solving it as a way of building brain cells. Use a smartphone or tablet somewhere in a comfortable position where you can browse the internet for a solution.
Use speech to text dictation. Get up and walk around while you are at it.
I find that doing a little computing at a time throughout the day and interspersing these sessions with physical activity works very well using a smartphone.
Imaginative new smartphone apps make it easier to do almost everything with your smartphone. This includes making slide presentations and both buying and selling. Easier on the eyes, too, than a large web page, as your eyes focus on a small area only.
A recent Mayo Clinic newsletter correlated sitting with shortened lifespan.