Baby Parrot
So you have a little hatchling. Baby parrots are born completely blind and naked. In the wild a baby parrot is fed regurgitated food by his parents. Human breeders will feed the baby parrot formula through a syringe injected into his mouth. The formulas contents will vary from species to species. But it should closely resemble the regurgitated food a parrot would get in the wild from his parents.
Imprinting Parrots
This is the stage in a parrots life when he begins to open his eyes and take in the world around him and identify his parents. They aren't picky, they will accept the first moving thing they see as their parent. But even though they might accept the first thing they see as his parent he also gets very attached to who he views as his parent.
This is an important time in a parrots life to get parrots accustomed to humans and keep them tame. That doesn't mean if you don't get a parrot in this stage of life he can't be tamed, but it will take training to accomplish that. Getting domesticated parrots from a breeder will hopefully gotten your parrot imprinted that humans are trustworthy.
Fledgling Parrot
This is the stage of a parrot's life where he begins to learn to fly or has begun to fly, but still depends on his parents for food. If a parrots wings are going to be clipped it should be done during this stage, but preferably after the parrot has already learned to fly. If you do it before, then he'll likely never take flight.
Weaning Parrot
As I'm sure you can guess a weaning parrot is in the stage of life where he's being weaned off of parent feedings. This is usually done once a parrot has completely learned how to fly. In captivity this is done by slowly introducing solid foods. This has to be done by a trained breeder so you don't have to worry about your parrot being malnourished or starved.
Juvenile Parrot
This is the time in a parrots life when he is able to fend for himself. Or in captivity is completely on a diet of solid foods and off of formula. This is the time a parrot can be sold. This is a good time to get a parrot of this stage of life because usually they are very accepting of new people.
You can tell if you are getting a juvenile parrot in most cases by looking at his feathers. It will of course depend on species, but most species of juvenile parrots do not have the full adult color and they sometimes don't get them until a few molting sessions.
Parrot Puberty
Yes, parrots go through puberty too. This is when the parrot becomes sexually mature and it can vary by species of parrot and even the sex of the parrot. There can be significant personality changes during this stage which might be short lived and might only happen during mating season. It's possible your parrot might fall in love with you and you'll see some strange behavior which includes regurgitating food at you or trying to make love with your hand or push his back against you trying to get you to mate. It's important that you not encourage this sort of behavior by stroking your birds vent when he does this stuff or handling him. Be patient with your parrot during this time and remember to keep him active and stimulated to help avoid some of the acting out you might experience when your parrot is trying to mate.
Adulthood
This is a parrot who has been through several mating seasons. He should calm down by this time and settle nicely into life as a parrot in captivity. But they are also less willing to try new things most likely at this stage of life. Kind of like many humans who get set in their ways in adulthood.
The stages of a parrot aren't all that much different from the stages of a human. They are born, cared for by parents, begin to recognize them, learn to walk, and eventually go out on their own. Going through puberty and then settling into an adult life and set in their ways. Parrot ownership is a lifelong commitment so be prepared for all of these different stages of life.