What's the oldest computer you have in ACTIVE use. It doesn't have to be every day, but not something that's sitting in a drawer you haven't turned on for a decade. Something you use. For me, it's a Lenovo X1 Carbon Gen 1, which originally was released in 2012. I'm currently running #Zorin 16.1 
on it, and it's perfectly capable of most of my everyday tasks.
@mike Mine's a Dell D630 laptop, running the Haiku OS. I use that chunky boy for taking notes in college 😆
I don't know how old it is, but it says "PTA Approved Model 2006" amongst the labels on the bottom, so... I'm saying about 15 years and going strong!
@golemwire Ooo, wow! Haiku! You don't run into many of those! I have it in a VM somewhere around here, but haven't fired it up for quite a while.