How old is my Ipad air 2 os 13.1.1?

No, you cannot add extra memory or storage to your device after purchase. You have to decide at the time of purchase how much storage you want and that is what you will have for the life of that device.

The current iPad OS is 13.7. You've skipped a number of updates if you are running iOS 13.1.1.

Can extra memory be added to my iPad Air 2?

No. Your internal storage is fixed at time of manufacture - and will remain the same for the entire life of your device.