Thing is I don't think it will age as badly as older models given all the power it now has. That is, unless iOS undergoes another paradigm shift.
I used to upgrade iPhones every 2 years because performance became unbearable on the 2nd major iOS update. I'd usually just update by one major firmware and then I'd get a new device for performance. The iPod Touch 4th gen, I actually kept on the original iOS version. Now that device was pretty RAM starved (256MB). The iPhone 5s? Still doing mostly okay on iOS 10 so that's 3 major iOS updates. It was the first iPhone I replaced because I wanted features on the newer models and not because performance was actually lacking.
Quite honestly, the iPad Air is a bit laggy but it's not unbearable. Looking back, the primary reason I was frustrated with my old Air was due to the paltry 16GB storage. Using it as a backup device to my 256GB Pro 9.7 whenever that one's charging, the Air is noticeably slower but not frustratingly so. Honestly, I think my mom would have been quite happy with the Air's performance over her iPad 3. However, she needed more storage than 16GB.
There's a reason decade-old computers can run Windows 10 just fine and that's because processors became fast enough for majority of the tasks that regular people usually did. Starting with Core 2 Duo, the bottleneck shifted to the storage subsystem so the best upgrade one can do isn't upgrading the CPU, it was replacing the HDD with SSD. For the retina iPads, I think Apple has managed to reach that point with A9 or A10. Perhaps even the A8X.