[capt_noonie]

This post makes me sad. It almost seems like he doesn't give you enough and are looking to put him down because of it. You say you "don't get much out of him" but what are you wanting to get out of him? At that age I would just be looking for companionship. Having him lay on my lap, or cuddle in bed for warmth. You didn't say if he has any illness, so I assume he is just old and doesn't play or walk.

If he's not facing any serious medical condition, I'd say it's not fair for you to put him down. I also think it'd be unfair for you to give him up to a shelter or rescue at this age. He is likely at the end of his life. He should live out his final days/years in the hold he knows and is comfortable in. But to answer your question, usually you will know when to put down is when the good days outnumber the bad days. How bad is bad, only you will know.