There is a down side to rescuing, especially with the parrots that naturally have shorter life spans. It is not only a matter of how old they are when you get them, but how good their care was for the first part of their life.
Some of my parrots are quite young, like Yo-yo, Frank, and Claudia. Yo-yo had a very rough early life, but as I got him when he was around 6 months old, those health effects were easily reversed (although he is still "underweight"from being starved, even if he gets a stick of millet a day.) Claudia's liver is a little weak because she was weaned far early, but she also escaped any other major, health-affecting, abuse. Frank has just plain avoided any bad homes, his only problems are behavior wise!
Some of my parrots are older, though, and from not-so-great homes. I do not know Chester's age, other than that he is at least 10, but I do know that the last eight years of his life before I got him were very, very rough, for both his physical and mental health. I am hoping that his young age will help him eventually recover from the physical (and mental, of course!) health effects; he has already greatly improved. Peter and Ava are both a minimum of 5, easily more, and are both showing their age. Even though budgies should be able to live 10-20 years, most of today's modern budgies (i.e, the "bin-o-budgie" or "box store budgie") are bred with such bad bloodlines, and often live is such poor conditions, that 5+ is not a very young age anymore, unfortunately. Yaz seems to be doing the best of these older birds; he is around 10, but still seems to be going strong (aside from his very closely shaved chest!)
The ones I notice the age and care factor in most are Miss Patty and Lola. I have no idea of Miss Patty's age, but she certainly does not seem very young. I know she has had many homes, and I strongly suspect in the first she was a "battery hen" breeder. When I got her, she was not doing well and seemed very sick and very underweight. She greatly improved with better care, but has slowly declined again in the past few months, and now seems rather fragile though happy.
Lola is over 12, really not that old for a quaker considering they should be able to live to 30 or 40. I do not know much about the care in her first home, other than that she had heavy metal toxicity when I got her, and smelled of cigarette smoke and laundry detergent (shower, anyone? She actually had to be "aired" with open car windows on the way home.) She also greatly improved, but this winter and Spring she has really started to show her age.
Both Lola and Miss Patty went to the vet in March, and were declared very healthy, all things considered, so at least I know that much. I try to give them lots of supportive care, which helps.
Despite all you do, all the good care you may give them, or be prepared to give them, some things can not be changed. I would not trade taking in these birds for a minute; giving them even a week of a better life is worth it, but it is so very sad. To see how wonderful these birds are when given that chance. To think that no one has before stopped to notice this, to even learn what some of their most basic needs are. And that these birds will pay in years for that neglect and abuse.