Humm, I installed Windows 11 on a really old Dell laptop (clean install). I'm sure it was not HW supported but it installed fine. I may have had to click something like, " Yeah I know it doesn't meet the specs"; but otherwise fine.
No, I don't like Windows but it's what my partner needed at the time.
As long as it is 8th gen Intel or newer it is officially supported. It depends on what you mean by "really old." I have hardware from the early 2000s that runs Debian.
It's a Dell Latitude D630 (model PP18L according to the label).
CPU is: Intel Core 2 T7250, 2.00GHz, 800MHz, 2M L2 Cache, Dual Core
Built: 27 MAR 2008 (actually newer than I thought)
Last OS to have support from Dell was: Windows Vista 32/64 bit
RAM is: 2.0GB, DDR2-667 SDRAM
You can run a bunch of of things on that hardware. The limiting factor is the ram so if you can upgrade it there will be a massive improvement. Also look into getting a SSD.
I would go Debian with Firefox ESR and ublock origin. You can apply the Firefox privacy patches if you want.