You know the funny thing is that of all the birds I ever had the tamest most trusting bird I ever had as a pet was an ancient wild caught adult Blue and Gold Macaw. Our vet, who was one of the top Avian vets in the country, estimated she was 35-50 years old when I got her. She had been attacked by dogs and found by an elderly couple in their back yard. The dogs had torn her up horribly and she was plucked almost completely featherless with a badly broken wing. The couple kept her for about a year. She had grown a lot of her feathers back, but there were several spots of open wounds remaining, especially on the broken wing, that still needed to heal. There were patches of skin missing even. She looked just awful, but was at least on the road to recovery when I got her.
We continued to nurture her back to health over a period of a couple years. She grew back most of her feathers, her wing fused together and just became immobile (she couldn't even extend it). Until she died years later, she continued to have problems with the feather follicles that would try to sprout feathers through the thick scar tissue. She fell completely in love with me very early on though in the process. She trusted me completely and wanted to be in my lap 24/7. I'm sure it was a combination of factors including that she was very dependent on me and she knew I had helped her get back on her feet so to speak (she eventually got around surprisingly well other than the feathers that were an ongoing irritation to her). She passed away a couple years ago after having been my loving pet throughout much of my 20's and 30's. I still get tears even thinking about her. I still even have her cage! It just goes to show you that you can bond with a parrot of any age. I really think the birds that are "hard" cases can be some of the most loving once they are in a good environment and feel safe there.