I haven't had the experience to see either of my MY dogs grow old to have to make this decision in the same way you are. Both at 8.5yrs and full of piss and vinegar ended up with cancers that were terminal. When I was looking into this for myself and struggling with the decision, I stumbled upon a vet's blog that basically said she never once heard anyone say they did it too soon, and only ever heard regret about having waited too long. A family friend also gave me advice that it's the last act of love and compassion we can give them. I am not judging you and say this with no intended shame at all, with only what you've said to go by, but I think it's time, and maybe a little past time. At the end of the day it's you and your well loved furry friend, not me and mine. It's going to be hard no matter when, so if you can, try to have family with you and planned support for a few days. You'll feel their presence for a very long time. My most recent was in April last year and that nugget is still around me in many ways, and getting blamed for my gas at both ends.
I went on a little unnecessarily. I'm sorry that this decision is upon you, but I'm glad you've been able to have and provide this companionship that has brought you here.