I understand your meaning above, jheem. I readily think of certain youths as being lively, nimble, brisk, and agile, but others as being anything but. However, I wouldn't describe an agile youth as being a spry one simply because my association of spryness with old age is firmly cemented for causes you suggested in your first post way above.

So, tsuwm, with reference to MW, we could legally get away with saying that Jack was a spry young man or Jet was a spry colt. Interesting. Denotatively, that is.