I have a 14 year old dog myself. He has cataracts and seems to be going deaf. However, he's still housebroken, a big plus. He also sleeps most of the time now. What I've done is mentally made a checklist of the things he does enjoy: attention from his people, eating, playing (briefly) with his babies. When he no longer enjoys those, coupled with health decline, then I will know it's time to visit that decision.
Xan Varik von Fernheim (born 4/3/13)