Meaning of the Oscar Wilde quote about the "tragedy of old age"

So far nobody has hit quite the right note on the quote. I would parse it this way:

Life is short, and nothing brings that home like reflecting on how little we have seen and done at the point when time is impatiently hurrying us off the stage. The tragedy is that we haven't really lived what we estimate to be full, long lives; for most, decline and death come way too soon.

It is the difference between the physical infirmity of age, and the mental desires that still exist. The giving up of cherished activities before we want to. The exclusion (or dismissal) by youth when we still wish to be included. The knowledge that one is reaching one's conclusion when there is still so much that we want to see and do.