I feed Orijen/Acana grain-free to all my dogs, two of which are currently 11 and 9. They are in fine fettle; you'd never guess their ages! What is your worry with a grain-free food?
I switched my 3 seniors to Orijen in October. Loved the results except for the weight gain on all three. They do gain weight easily but they seemed to gain alot more on Orijen. I cut their portions quite a bit from what I was previously feeding but they each still gained alot of weight. Is this normal for grain free foods even if you reduce the amount you feed?
You do feed less grain free to maintain proper weight.
A lot of dogs will actually loose weight on grain free.
The suggestion of thyroid check is a good idea.
The higher fat in grain free can be a problem for less active seniors, causing weight gain. Sometimes dogs don't digest fat properly causing weight gain. Lecithin can help. Talk you your vet about it. I used 1200 mgs daily and it worked for my girl. Her issue was found thru blood work.