For a lot if what I do, the CPU on an older laptop is just fine. So, I too use really old laptops. That said, I am upgrading laptops that sold with 2 - 4 GB of RAM to 16 GB, or at least 8 GB. Often I am replacing on old HDD ( maybe as slow as 5400 RPM ) with a faster SSD. That SSD also juices the battery life. So, what I am using is a lot nicer than what the original owner had to deal with. Many laptops today will no…
That said my daily runner is now T420 from (2011-ish) that came with 6GB of RAM that is in near mint condition I bought for $100. I swapped the HDD for an SSD and just threw Linux Mint on here - I disable the swap file so that it doesn't hammer the drive. So when it is 6GB of RAM - that is all it can work with but it has never been an issue.
But the original HDD was running Windows 7 and oh my gosh the bloat they had installed! This poor thing was in pain with that junk on board! It is amazing how many times I have had an old laptop come up, unusable because of how the OS treats the system and the users just keep dumping more on it that is reasonable. But swap the disk drive and OS and you would never recognize some of these things.
I once came across some netbook Toshiba Presario (?) - 1.4Ghz Core Duo with 3GB of RAM running Windows Vista. The poor thing! But do the old one two on it and it was a brilliant little machine for plucking away on, could easily get 6-7 hours on battery as well.