Agriculturally speaking, there really is a creature known as a spring chicken, although chefs may call it a Cornish game hen or Poussin. Bred primarily for eating, it is a very young bird with a high ratio of white to dark meat. The meat is said to be much juicier and more tender than older chickens raised for the dinner table. During the earliest days of poultry farming, it was impossible to raise chicks during the cold winter months, so a chicken brought to market in the spring was prized for its youth and fresher flavor.
Metaphorically speaking, a spring chicken could represent a person in the prime of his or her youth. Such a person may be a little naive or unseasoned at times, but he often makes up in physical agility and personality what he may lack in worldly experience. A young college student may be described with this term by others who envy his or her youthful appearance or unclouded worldview.
It is often more common to see the negative form: "no spring chicken," in print, because the speaker wants to emphasize his or her non-youthful status. A character may apologize for his or her slowed performance, citing the fact that he or she is no longer young. The reference is more of a self-deprecating remark than a criticism of younger people. Being considered a spring chicken by an older person is not usually taken as a pejorative, but rather as a comment on one's youth and vitality.
Some restaurants still promote the freshness of their poultry by calling it spring chicken, although modern advances in poultry farming make that designation less significant. The same features which made the original young bird so prized can now be duplicated all year round. Chicks can be hatched in staggered batches and housed in heated incubators, which means a chicken under the age of 10 months should be ready for market at any time of the year, not just the spring months.