There are several things that could cause a sudden shutdown like this, and age is certainly one of them. But heat and power come to mind first for me. Is the interior clean of heat trapping dust? Do all the fans spin freely? If clean and the fans work, and it still does it, you might blast a desk fan into the open side and run like that for a while and see if it happens.
You need to ensure the power supply is still good. PSUs "age" faster than low voltage devices, and if any hardware has been added since new (RAM, for example), that PSU may be straining, and beginning to fail. The best, conclusive way for most users to do that is to swap in a known good spare. Otherwise, here is my canned text on testing PSUs.
To properly and conclusively test a power supply unit (PSU), it must be tested under various realistic "loads" then analyzed for excessive ripple and other anomalies that affect computer stability. This is done by a qualified technician using an oscilloscope or power supply analyzer - sophisticated (and expensive) electronic test equipment requiring special training to operate, and a basic knowledge of electronics theory to understand the results. Therefore, conclusively testing a power supply is done in properly equipped electronic repair facilities.
Fortunately, there are other options that are almost as good. I keep a PSU Tester in my tool bag in my truck. The advantage of this model is that it has an LCD readout of the voltage. With an actual voltage readout, you have a better chance of detecting a "failing" PSU, or one barely within the required tolerances as specified in the ATX Form Factor PSU Design Guide (see “Table 2. DC Output Voltage Regulation” on Page 13). Lesser models use LEDs to indicate the voltage is just within some "range". These are less informative, considerably cheaper, but still useful for detecting PSUs that have already "failed". Newegg has several testers to choose from. However, none of these testers test for ripple and they only provide a little "dummy load", not a variety of "realistic" loads. So while not a certain test, these testers are better than nothing. They are also great when using a spare PSU for testing fans and drive motors as they signal the PSU to turn on when plugged in.
Note the required voltage tolerance ranges:
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Swapping in a known good supply is a tried and true method of troubleshooting used for years, even by pros. If you have access to a suitably sized, spare power supply, carefully remove the suspect supply and replace it with the known good one, and see if the problem goes away.
I do not recommend using a multimeter to test power supplies. To do it properly, that is, under a realistic load, the voltages on all the pins must be measured while the PSU is attached to the motherboard and the computer powered on. This requires poking (with some considerable force) two hard and sharp, highly conductive meter probes into the main power connector, deep in the heart of the computer. One tiny slip can destroy the motherboard, and everything plugged into it. It is not worth the risk considering most multimeters, like plug-in testers, do not measure, or reveal any unwanted and potentially disruptive AC components to the DC voltages.
Note the ATX Form Factor standard does not "require" specific color coding for power supply connector wiring. It has recommendations but manufacturers often do not follow them. Sadly, many testing guides or tutorials will refer to wire color only and that can lead to improper testing.
The voltages can be checked in the BIOS Setup Menus of most motherboards but they do not reveal ripple or other anomalies either. And of course, booting into the BIOS Setup Menu requires a working PSU.
As always, before working in the interior of the computer case, take necessary ESD precautions to ensure static buildup in your body does not discharge through and destroy any sensitive devices. Unplug from the wall and touch bare metal of the case before reaching in. And remember, anything that plugs into the wall can
kill. Do not open the power supply's case unless you are a
qualified electronics technician. There are NO user-serviceable parts inside a power supply. If you do not have a tester or a suitable spare to swap in, take the PSU to a qualified technician for testing.
For more information on testing PSUs, see this excellent article by Gabriel Torres,
Why 99% of Power Supply Reviews are Wrong.
If you've kept the interior clean all this time, the temps are good and the power is good, then it could be a component on the motherboard, or just about anything plugged into the motherboard. That's what make sudden shutdowns, freezes and reboots so hard to troubleshoot - all sorts of things cause similar symptoms.
In the meantime, assuming the interior is clean, you might backup any and all files you don't want to lose.
Thanks again for your time and advice.
Kind regards,
Declan.
Good luck and sorry I could not be of further help.