My biggest fear on a personal level is that I would adopt the bird, fall in love and he/she would only live a short time. Preparing myself for that going in I would give the bird the best quality of life that I could for the time he/she has, which could be decades like you said. The younger (nicer?) bird probably has a better chance of being adopted by someone else since most will be unwilling to take on the older bird. I admit, I did not get a rehomed bird. I got a baby. I GREATLY admire those who are willing to provide a happy home for these birds who are in need more so than a baby.
If you scroll down to the thread called "Rescue Room", you can read lots of stories and experiences from those who have adopted previously owned birds.