I'm mostly in agreement with coffeeking on this (though a bit less of the anthropomorphising the hound). If dog's happy in himself and not in pain or any suffering, or has no signs of imminent demise I'd keep him until he gets to that point. That might be now, or might be when his eyesight goes to the extent he can't navigate round things, or when legs go to the point he can't get around the house or go for a big steaming poo in the garden.
Only you can tell when that time comes. Then you can be glad he's a dog and not a person so can be put out of a miserable existence with a bit of help.
If you think that the time is now then you know what to do, but I'd seriously reconsider doing it without discussing it with the missus. It's upsetting to say goodbye, but gives a sort of finality which makes it easier to come to terms with the demise of the pet. It's not the sort of thing you can undo, and might help you decide what to do. When the time came for "our" dog (very different circumstances so as easy as those decisions get), even though it was really my gf's dog (and the dog thought that too!) and she'd had the dog longer than I'd been about we made the decision together - I'd have been gutted if she'd gone it alone.