Just like how evolution has given rise to the species-typical differences that might help us interpret the behavior of an individual parrot that we discussed in the last segment, it has also resulted in behavioral differences between different life stages. Behavior always happens in a context, and the life stage (ontogeny) of an individual is very much a part of that context.
Sometimes a behavior only makes sense at a certain point in life. Fledging is one example. As a baby parrot is growing, before it even has fully developed feathers, it will periodically flap it's wings in the nest to start practicing the necessary stamina and motor skills they will need later. Fledging as quickly as possible is likely to increase the survival of parrot chicks in the wild, so it makes sense that this is something parrot chicks at a certain age are very motivated to do. Being late to that party can have fatal consequences.
Once the young bird has fully developed feathers, they are ready to take their flight practice out of the confines of the nest. Learning how to fly is hard. Luckily, young parrots have millions of years of evolution behind them to make this learning process as smooth as possible. Accidents can definitely happen as they learn how to navigate this new 3D world, but they are rarely serious, and they learn fast.
In contrast, if a young parrot is not allowed to fledge and develop good flight skills at the time they would leave the nest, this becomes a very different deal later on. In the wild, it makes sense to practice and play as much as you possibly can when young, to build the necessary flight skills for life as an adult. However, once grown up and on their own, many species become more concerned about conserving energy than spending it on frivolous flights, at least if food is scarce.
Teaching older birds to fly is (generally, far from always) possible, but it is an entirely different and much more difficult deal.
Play behavior is also influenced by life stage. Generally, across the board no matter if we're talking mammals or birds, play is more common in juvenile or young individuals than in older ones. This doesn't mean old birds don't play, but young ones are certainly more pre-disposed to, and they tend to do it more often.
Breeding is another life stage that influences behavior a lot. Reproduction is very serious business if you are a parrot, and an adult bird in a reproductive state is very different from one that is not, both physiologically and often behaviorally. A breeding bird will likely be interested in different things and it is likely to show a range of behaviors and body language that you might not see in the same individual when it is non-reproductive. Being in reproductive-mode doesn't necessarily cause these different behaviors, but it is, again, a part of the bigger context in which the bird is behaving.