Above pic was visting the mealy amazon at the shop owner house.. seeds diet is a headache way to covert them to pellets.. stress to birds..
So above pic hard to tell the age. But roughly the age???
Ya, it's a pain for some birds, but millions of bird owners have successfully converted their birds off seed diets. Our amazon eats a fresh foods diet and while it was difficult to convert him, he is very used to it now and eats healthy foods readily. Also, it is not impossible to train an adult bird to wear a harness. Our amazon, who we rescues at around age 10, is now around 18 y/o and slowly becoming accepting of his new harness

Patience and persistence is often necessary with parrots, whether young or old. They are not domesticated animals, even if the individual was captive bred, and their instincts still run on the wild side. Training them to do just about anything can be tedious, but entirely possible for the majority of individuals.
Always difficult to tell age of parrots, but the bird looks like a older juvenile to me. Likely over a year old, but likely not older than 4-5 and likely not sexually mature. Just has that 'baby' look about it still. Did the shop owner give any indication of age? Also wanted to mention, the environment the bird is in (judging from the photo) looks very clean, orderly and the bird looks to be clean and in good health as well. From what I can see of the other cages, those look well kept and clean too. I know there are differences in different countries on how pets for sale are kept, but I wouldn't be at all concerned about buying a bird being kept in those conditions. Seems the pet store owner takes good care of his/her birds.