Honestly... A Power supply does degrade constantly after purchase... Aka. Cap aging.

However, that is entirely dependent on how significant the loads placed upon it are over time. If you have only placed moderate loads on the PSU, it is likely still fine.
I would replace it when you do your next round of upgrades/parts replacements.

The PSU of my old PC was 15 years old, when it died two months ago... the usage of that PC was way above average... 24/7 in some years.

That's why I'm questioning it. I do 'light tasks' at least 90% of the time and only a few hours a day on average, I do most of my gaming on consoles, so it could be fine for many years ahead. I changed it's cooler to a custom one around 10 years ago but I don't even remember why - either it had stopped working which I doubt, or I wanted a quieter one.

How long it will last depends on a bunch of things.

Quality - Higher quality will very likely last much longer. Cost usually means higher quality but not always. Heavier usually means higher quality but not always.

Run Time - The more you use the device, the quicker the PSU will degrade. Less usage usually means it'll last longer, but not always.

Temperature - The hotter the PSU runs, the quicker it will degrade. The cooler it remains, the longer it should last, but not always.

Consumption - The more power you draw, the quicker the PSU will degrade. Less draw usually means it'll last longer, but not always. Right around 50% of the PSU max wattage is the most efficient point of consumption, when the PSU is new. As it ages, the peak wattage degrades over time.

Environment - Dirt and dust build up will reduce airflow and increase PSU temps, so use compressed air to clean it every so often. Higher outdoor temps will lead to quicker degradation without better or additional cooling. Dampness and higher humidity will cause quicker degradation, in which case better airflow, some extra dry heat, or dehumidification will help. Being near the ocean can cause quicker degradation, in which case you'd want to keep the house/room closed off from outdoors with well filtered air. 

Surge Arrestor - If you only use a direct plug in or power bar, a large enough power surge could cause a PSU failure. Just any old surge arrestor isn't enough because it likely won't be able to handle a very large surge or won't sustain many surges. Typically a higher cost arrestor will be able to handle more, but not always.

Reviews - If you can find tests and data on the exact PSU, multiple sources if possible, you're more likely to be safe, but not always. PSU models can have internal parts changed over time, so a review isn't always spot on if their unit had some different parts in it.

It's a lot to take into account and some of those solutions aren't really viable for some people, beyond going even more extreme for the most hardcore.

No matter how well you maintain your system, just the right situation, or combination, can kill a PSU. The more you take into account and more overboard you go, the less likely you are to have a failure. Less likely doesn't mean impossible, even if it's fairly new.

This unit was nothing special back around 06/07, kinda no name, yet I used it in a Q6600, 8600GTS build. I quite often pushed the PC while gaming as hard as I could for the first 3 or 4 years, then I used the PC very casually until 2020. That PC had pretty decent cooling. The PSU still lives and never gave me issues, other than having to replace the 2x 80mm fans that were on their last legs around 2015. I built a new PC in 2020 and went with a new PSU just because, plus it was a great price at the time, but I keep this one as a back up just in case.

I'd guess your PSU is still fine, assuming you don't ever plan on pushing it too hard, too often going forward. Your 700 watt unit may only be good for 600 watts, maybe 500 worst case now. If your PSU is of relatively decent quality, it's quite likely when it dies, your PC just won't boot one day, but there is always the slight chance the PSU dies and takes out one, if not multiple other components in the system. Though a brand new PSU, while unlikely, could do the same thing anyway.