Graeme Quirk explained the origin of the term ‘spring chicken’. He says that it’s all to do with a chook’s breast and thighs.
In the past, chickens were only available for eating in spring. Spring chickens had to be less than 10 months old with breasts and thighs that were fully developed, but still soft and juicy enough to eat.
In humans, the term ‘no spring chicken’ means no longer young or agile.