4th Gen iPad – 5 years old. It was the first one that came with a lightning cable. Still works fine, although it seems that some apps are released requiring technology that this iPad doesn’t have.
With regards to shelf life, I’m sure Apple just aren’t sure when they release something when it might be obsolete. I would imagine new iPads/tablets contain tech now that hadn’t even been thought of when I bought this one.
If anyone has a slow tablet it might be worth backing it up, restoring to factory settings, then reinstalling from the back up. I did this with a iPhone a few years back. Made a huge difference.