I agree with everyone else - unless the vet mentioned some health issue, age alone isn't a good reason to stop riding a horse
I agree with Ella, get other opinions.
if you board at a barn with a few trainers ask their opinions and ask other vets if you can.
I would see at least a couple more vets and get their opinion.
If you feel that your mate is perfectly fine in walk, trot and cantering then I'd say keep doing what your doing, hope this helps