[credo]

My older iPad (yes, first gen iPad from 2010) is eight years old and it works great (from my perspective :)

Yes, I can't download most new apps, but last year, I released an update for 'Economy for iPad' (#1 Finance app for several weeks in 2010) and that app continues to support the original iPad and iOS 5.1

Generally, the best way to keep old devices running well is to keep the old OS. (Old major version at its latest minor version e.g. 10.3). There is a slight risk of not being able to pick up security fixes that didn't make it into 10.3, but made it to 11.0. However, as a developer, who tests apps on older OS versions, I keep old test devices with old OS versions on them.

Btw I should also add that Netflix runs perfectly on my old iPad with iOS 5.1 :)