Despite computers' reputation for becoming obsolete even before you buy them, newer computers do last a long time. In fact, they can last long enough so that there's no clear point where they have to be replaced, unless you know what to look for. Simple but chronic issues are the key to telling when you have to replace your computer.
It's not unusual for a computer in otherwise good condition to have what's known as a kernel panic, which is an internal system error that requires the computer to immediately shut down. If this happens once in a while on your computer, you're likely fine and just need to restart. But if you start having kernel panics a lot — daily or even a few times a week — then you need to start looking at replacement computers. Repeated, frequent panics mean your system is becoming so corrupted that it can't function properly.
This may sound like an unusual sign because fans are among the simpler items to replace in computers, even laptops — so why bother replacing the entire device? Simpler doesn't always mean easier, and some computers require time away at a factory to get the fans replaced. If one fan breaks but you have another that works, you might still be OK. But if both fans break and you can't afford to lose access to your computer for even a few days, your computer is not going to work very well. It will keep shutting down as the internal temperature rises. You can stave this off a bit by aiming a small external fan at the computer, but that limits portability for laptops and can be rather inconvenient for desktops, too. It's best to look for a new model if replacing the fans is going to be too inconvenient for you.
Upgraded OS Makes Your System Clunkier
Keep a computer for long enough and you'll find you have to upgrade the operating system (OS) at least once or twice. The upgrades are usually made to be fairly seamless across a couple of versions, but as you start upgrading more and more, you'll find that your older hardware just isn't cut out to handle the newer OS and software. When you reach the point where your computer still works but has slowed down and doesn't display some screens properly thanks to growing incompatibility between your hardware and OS, it's time to look at new models.
If you're just starting to have problems like these, contact a repair shop that offers Mac support, such as Computer Help, Inc, to find out if the issues are fixable or if they're ones that require more detailed action.