[93n]

I still run, and am writing this from, a 16-or-17 year old Dell E6510 (i5-540) that serves as my "around the house" computer. The battery life (with a huge 9 cell that protrudes out the back) isn't great, and it's hot and heavy, but with 8GiB of RAM and an SSD it works pretty well on Trixie with Cinnamon.

My main machine is a 13 year old Lenovo y510p running Debian and KDE. You'd be hard-pressed to tell that it's anywhere near that age.

The only aspect that has been annoying with both is graphics. Both machines are nvidia and are long past their support periods. The y510p has SLI (one graphics card is in the CD drive slot) which never worked on Linux. When removing the second card, the on-processor Intel graphics can be used, which have better support than nvidia, so I stick with that. I don't do anything graphics intensive anyway.

The biggest upgrade with old computers, without any doubt, is an SSD. I still remember getting my first one back in about 2011, a used 60GiB OCZ Vertex, and it was truly magic seeing the computer boot to the desktop in a few seconds even on a core 2 duo.

[ColdStream]

I am in the same boat as you. I run a T400 from 2009. It would have been fairly high end for the time (2.4Ghz Core 2 Duo, 4GB RAM) and I think that is why it has held up so well.

As now, there is a very wide performance profile for machines around then. From sub Ghz Celerons up to 3Ghz plus dual core things.

[93n]

I'd expect your T400 is similar to my E6510 in terms of construction since they're both business machines from the same era. The E6510 is a tank. The hinges are still solid and there's no wear on the keyboard. There's a single panel, held on by just a single screw, that pops off on the bottom to expose the majority of user-replaceable parts.

It'd be useless with Windows, but works great with Linux. The only issue I have is severe lag on google maps.

[93n]

Tangentially, my home server is a slightly newer NUC with a toy of a celeron N3050. That runs great for the overwhelming majority of use cases, too. The only obviously slow operations are anything heavily CPU-bound, like compiling (esphome) and photo indexing (photoprism). In the latter case, I run it at night on a cron, so it's not really an issue in practice.