A signed vet certificate should accompany the bird. If all the tests and the exam reveal no apparent disease you can feel fairly confident about your purchase. As a precaution all new birds should still be quarantined from the rest of your birds for a minimum of 45 days. The age is less important than the health and temperament of the bird. Buying a young bird does not guarantee tameness and buying an older macaw does not mean the bird will not make a good pet.
Macaws are individuals.
Macaws will continue to grow until they are about a year and a half old. Baby macaws have dark eyes which lighten as they age, except for Buffon's and Hyacinths which remain dark when they are mature. The gender is not important for pet purposes. The only reason to desire a certain sex would be to breed or prevent breeding. Some pet owners may desire to get birds that are the same sex as the birds they already have to avoid breeding.