In all honesty I don't see why you'd need to do this. We have a computer from 10 years ago.
Need to for basic web browsing and communication tasks? Probably not.
On the other hand, if you spend a significant amount of your recreation time/budget on computer games, if you like managing all of your media digitally, or if you have take-home work from your job or school that requires you to be able to access the latest versions of popular productivity software on your home PC, then you're looking at probably needing a new one every few years or so. Of course, two of those are luxuries/hobbies, and the other may not be something a person would have to worry about after retirement, but still, there are some reasons for people to upgrade on a regular basis.
Photography is a hobby of mine and I find myself buying anew computer ever 3-4 years. Of course that was before my forced semi-retirement. Now I will think long and hard before replacing my electronics. My cell phone is still paid by my employer and with data it runs about $80 per month for my iPhone. I used to think nothing of buying new software and apps, but now I make do with as much free stuff as I can find or I find that I really don't need the convience of a costly product. I love the latest and greatest in technology, but I know that the build up before I buy is greater then the satisfaction of owning it.