>I can't imagine referring to someone youthful as being spry.

but they usually are. the *attributes actually assigned to spry (Lively, active, and brisk; vigorous [AHD]) are so often thought to be youthful that the word drifts into those usages you think of. It wasn't always thus, and modern dictionaries don't mention that parameter (M-W has it as synonymous with 'nimble' and 'agile').