Man that's an old dog!! I'm guessing 10 years give or take, depending on how much sand,gravel is in his range. He has considerable wear on all teeth and you did him a big favor sending him to the big rabbit patch.
Whenever I kill an old dog I often wonder all the adversity he's endured to get to that age and all the close calls he's survived. I agree that it looks as though he may have been in a trap at some point and was lucky to have escaped that predicament, those front teeth are not only worn but some are broken off.
