I repeat this story every now and then but I "maintain" a 18 years old laptop with Ubuntu (mainly for Internet) for non-tech savvy user. I put it in quotes because I just run apt update every now and then - that's it. Just works. The only bottleneck is how resource-hungry browsers got over time but it remains usable. Ubuntu was installed sometime back in 2017 and there was no need for fresh reinstall since then.
That can't be literally true, no release of Ubuntu is still getting updates after 18 years. At some point you have to upgrade to the next release, and that's not quite as simple.
I did that for my mom. At some point she learned to click through the Ubuntu updater and she kept her machine updated by herself. I only kept tabs on her computer via the server monitoring tooling I had on my network.