my vet did my section a and placed him in the 7 - 10 age bracket. When I rung the passport agency to find out their feeling the lady on the phone said to pick an age in the range given by the vet!
lady doing a passport for a pony on our yard aged him at 5, as did i going by his teeth. the owner didn't like that answer so asked for it to be put as 10. which it was.
A passport is not classed as proof of age which is why the agencies are a little less worried about what people put on them!! I would put a happy medium between what the Vet has told you.
Angua2: Your Vet ought to be able to tell a 7yr old from a 10yr old due to the fact that the Galvayne's groove is not usually visible at 7 but appears at 9 onwards.