What a handsome little guy he is! Looks well cared for and in good health too. Just my opinion (and lots of experience with zons), he doesn't look like a juvenile but he can't be that old either. His plumage, nails and beak are that of a bird 5-40 years old. It's just a subtle thing, but he doesn't have that "baby" look about him. He also doesn't have faded plumage, thin patches, overgrown "gnarled" nails and beak or visible eye problems older (40+) birds tend to have due to age. His head color is also still confined to just his head with a little patchy spread down his neck. With most amazons, their head colors will spread down their neck and throat as they age, and by 20-30 y/o, MOST will have rather solid head color all the way down. As I said before, you'll probably never know for sure, but it's probably safe to assume the prior owner was spot on saying he was around 10. He looks like a younger, sexually mature bird.
And as just a point of reference, if you compare (yes I know their different species, try to look beyond that) the quality of plumage, nails and beak with our roughly 15 y/o amazon, you will see a lot of similarities. If you look close at some other folks profile pics who have older birds, you will see what I'm talking about with faded plumage and variations in nails and beaks of older birds. Regardless, best of luck to you! He looks quite feisty lol.