Yep it doesn't matter how old they are or if it is a peahen or a peacock they can and will display. Even if the peacock doesn't have a long colorful train and he is just a yearling he will display and not only that but he will act like he is hot stuff. My peahens mainly display for 2 reasons...1. Sometimes they just feel like displaying like if a peacock is displaying they might want to show off too. 2. If they see danger or something they want to scare off they will display to make themselves look bigger and scare whatever it is be it a dog or a squirrel.
From watching my adult peacock, he displays most around breeding season (of course) and after he sheds his train he doesn't display for a month or so (anyways he has barely anything to display) then when his train is about halfway grown out he starts displaying and from there he displays more and more. He also prefers to display early and late in the day, not so much in the heat of the day that is when he is lying around.
Peacocks display to attract peahens. They shake their feathers and turn. Each peacock has his own way of showing off, some do it better than others. Then the peahen will walk near him, pretending not to notice him. She is really checking out his display. Peahens have different tastes probably but they look for a healthy peacock. Are his feathers of nicer color and quality? Is his train big? Does he have a lot of eye feathers? These are probably some of the things peahens might look for that signal that he is healthy. Peahens will look at several peacocks probably before deciding which one they will mate with. Of course in a pen with just one peacock the peahens can't have a selection but the peacocks will still display to get the peahen's attention.