I have a friend who visits about once a year. I have a standing request that he brings his "Daily Driver" computer with him when he comes.
He calls it a tune up. What I generally do is go through the machine, run a virus scan, uninstall spyware, and send him on his way. It runs much faster because I've cleaned out the junk.
He's also been using that machine for longer than even I have expected. He's gotten newer machines, but he keeps coming back to that beast of a 17 inch "laptop" because I'm able to keep it going.
Eventually, he'll have to stop using it, and then it will have a second life as either a table leveler, something to hold a shelf down in the linen closet, or I'll put Linux on it and it will be good for another 5 years of use.
I'm leaning toward Linux, but that is because I actually do like using the environment.
Computers have a longer life than the manufacturers want you to believe because they exist to make money by selling you new. It's Planned Obsolescence.
With a phone, it appears much more clear cut. Especially with a smartphone, things have a shelf life. The vendor puts out a new model, it can do more, but does it really warrant you getting a new one?
Again like with my friends beast of a laptop, to me, it appears that it is software driving the decision.
There are two schools here. Apple and Android. Not looking at this as a fanboy of either set up, I have a preference for Android because I can do things with it like use the phone as a multimedia computer much easier than I can with iOS. I look at it as a use case to form a decision as to which works best for me.
Your Mileage May Vary.
With Apple, there is a clear end of life with their phones. When you can no longer run their current operating system, it is time to consider moving on. Apple has always done this with their computers as well. For a while their PowerPC computers were supposed to be the best thing out there. Then they came out with Intel based computers that made their old computers look horrible and they stopped supporting them after one more upgrade.
My iPhone is an old 3GS. It will still make calls, but as a computer, Apple is actively pushing it away. I have software that ran on it until I updated it, then all the sudden the older software is gone, and the newer one doesn't work because I don't have the current operating system. One after another app is going away and eventually that will be the end of it.
Of course if you have the latest iPhone 6, it's obsolete when you drop it on the ground on the first day it's out because you just broke the screen.
Android is a different animal.
Android support varies with the company that made the phone or tablet. Typically, an Android phone will get updates within the operating system version that it was bought with. After that you are on your own.
My tablet, a Samsung Galaxy Tab 2, got updates until the current OS came out. That doesn't mean that the tablet is unusable, it merely means that it will get more behind the times as I run into the same problem that my old iPhone had. Software won't be written for it.
There is another problem with the older versions of Android. The browser that shipped with every version of Android except the current one has a rather nasty bug in it. The short of it is that if you have an older Android device, do not use the default browser. Disable that browser, and install another. I did that at the start and I use Firefox which is the suggestion that is made by most security groups.
Why is that a problem? Because if you don't have a current device that runs the current Operating System, you aren't going to get an update and you are on your own. That means you have just hit the wall with using that phone, it's now obsolete - if you want to be secure.
It all seems a bit alarmist, but considering how many people use their phones and tablets as their main computing devices these days, it really does pay to be aware of what that device is capable of doing. It is a computer and they do need to be kept up to date. But when you can't do that any more, you have to be aware what not being up to date can mean.