All computer age. It is a fact. This year, its top desktop computer, eighteen months later is average. Wait another eighteen months and it will be poor. After that, they may work but its worthless if you try to sell it.
If we are discussing laptops, it probably has about three years of life. For a smart phone, it's even worse; I count on about two years. There are a couple of reasons for this, but mainly with me, it's because I carry them around, despite all my efforts, they get dropped, water gets split on them, and often I leave them in hot places like cars and my pocket. Heat is the main enemy of electronics. Lastly, even if they last they go out of date and unlike a desktop computer they cannot be upgraded
For businesses, I think its best to plan on a life cycle of about four to seven years. Once the computers start going over four years, its time to reconsider what to do with it. The big issue in business is that old computers run slow and employees are paid for time. A shop assistant costs you $19 hour, once you include holiday, sick pay and superannuation, that comes to about $24 an hour and that is the cheapest you can get. Currently if an old computer say without a touch screen or an SSD drive costs an employee about 10 minutes more a day. Over a year that works out to costing you over a $1000 for that employee per year just in salary. Now add into the equation, that many people work on overtime rates, many senior staff members spend more time on the computer so they lose more time and they cost more, your customer service is 10 minutes per employee worse a day, your own time etc. With probably even the cheapest employees from a business point of view, a computer upgrade can be justified as it's false economy to hang on to old computers in business.
These of course are only averages, sometimes you need to replace them much earlier as some are just lemons.