spry

From the Britspeak angle (Angle angle? ) I don't hear "spry" used all that much, but I believe it does mainly relate to vigorous oldsters. You'd talk about a "spry old chap", and never about a "spry young chap". A quick glance at the dictionary tells me the etymology is uncertain, so you've found an interesting word here, Geoff.

"Senior Citizens?"

We definitely have Senior Citizens, the term often being used alongside OAPs (Old Age Pensioners). e.g. entry to a high-adrenaline Theme Park would be 30 quid for most people, but 15 quid for OAPs and Senior Citizens (for the ride of their life ).
I suppose Senior Citizen implies someone of an age where they could have retired and could be in receipt of a pension, but may actually still be working, or perhaps have come into an inheritance, so are unable or don't need to draw a pension.
Also interesting.

[think bubbles in a goldfish bowl...]