When is it time to say goodbye

by brad
(California)

Our JRT is almost 17 years old now. He's been an amazing dog but time has certainly taken its toll on him. He can barely hear now and his eye sight is failing as well. He cannot go out in the sun because the brightness bothers his eyes now. He is healthy according to a recent vet visit although the Vet mentioned that he may have some pain 2ndary to arthritis. He doesn't come to us anymore but will follow us around the house at times. He basically isn't social anymore but will still bark and chase his favorite ball. He is pooping and peeing in the house and vomits water after drinking too much. He barks for no reason at walls and chairs at times. He's really becoming difficult to care for because of his senility. When is it time to put your best friend down---when we see him play with the ball and do a quick step back into the house after a walk it really makes you think that there is still a spark but he wanders aimlessly at times around the house and doesn't like us to pet him much. He walks away from us?
It would be so much easier to let mother nature take its course but is that the answer in this case? The vet thinks he could possibly live another year?