I expect you will probably redefine useful by the time you hit 60, or is it actually your belief that senior citizens are useless?

Probably. Note: this is not a rational belief, just an emotion. I feel that for me, my useful lifespan ends at around 60, meaning I have less than half of that to look forward to (and all of that on a downward quality slope). Objectively, I understand that there are important roles for senior citizens, and I do enjoy the company and services provided by many of such people. Just that trying to imagine myself getting to that age fills me with despair.