It's too old when it no longer serves it's purpose, much like us mortal humans ;-)
My wife and I each have a 2011 MBP running High Sierra, neither of which can be upgraded any further. For the most part, they still work, but over the last year or so we've been picking up compatibility problems with the latest Adobe apps, among others, as well as some occasional hardware issues like wifi and Bluetooth dropping out. I've also been getting nervous about online security with an OS that's no longer supported.
So we each have M1 Air's now, but these new machines don't run iPhoto, Aperture or iTunes, but the oldies still do perfectly well. Those machines now stay at home as offline 'desktops' connected to 24" monitors, if we need to use things like Aperture. I also still use my 2011 for digitising vinyl records and playing music via iTunes because it now mostly lives right next to the Hi-Fi.
I must confess, we've happily dragged these old machines all over the world for the last 10 years, and we got used to it, and they have been thoroughly abused along the way and survived, but once you start using an M1 Air you'll have a hard time going back to a cinder block like the 2011 MBP ;-)
As already mentioned, if you have decent backups in place, hardware failure isn't something to be worried about.