I think a senior Quaker is 14 or 15 years, maybe a few yrs older? It depends upon how well the bird has been cared for (diet, exercise, etc)and whether it’s had any injuries. I think senescence or old age occurs when the bird starts getting arthritis, cataracts, scaly feet, rough feathers, other signs of age. Quakers are prone to fatty liver disease and need careful diet and exercise.
My QP Lucy passed at 24 yrs but I and the vet considered her ancient as she had lived most of her life with one eye, a lame leg, and unable to fly. She had gotten more and more tottery and fragile in her last 3-4 years, when she rebroke her left leg (at 19-20) and lost some toes. She had had a liver issue (related to egg yolk peritonitis) at about 7 yrs but we saved her. After her early life injuries, Lucy was cared for well. I think Lucy did very well to last that long, and if I had not had a medical background, she would not have. She needed lots of intervention in her last few years.
Willow QP is 8 or 9 years old but he is in fine health and the prime of his life. I hate to think he will be a senior at 15 but I may well consider him a senior then. I would certainly say a 20 year old Quaker is a senior. Willow appears, on careful assessment, to be a young bird. Honestly, old parrots begin to look grizzled. Scaly of beak and foot and hoary of feather.
I’ve been a QPS member for many years and there are many Quaker fans there. You could contact the Quaker Parakeet Society (QPS) for other opinions.