A really young bird will barely have the classical white ring and head feathers will not be as vibrant. Like jay said, age of a pheasant is usually determined by the length of the spurs for us layman.

Unless your planning to send it to a taxidermist, any pheasant you shoot is a good one. I once shot a pheasant with spurs almost an inch long. I had it mounted only to have our goldie tore it up.