Our black lab had a good innings lived until he was 17 but i think he began to loose his marbles, the turning point for me regarding the vets needle was the day i came downstairs and he growled at me. He'd not done that in an aggressive manor ever in his life. Also this was one of several rather out of charecter issues, he'd tried to attack my brothers rucksack!
I then knew it was time for him to go. Now i'm not suggesting the situation is the same for you, personally i hope not because we do get very (perhaps too?) attached to our dogs.
Cheers
Aled