I had a roommate with a dog exactly like this. he was ancient looking when she adopted him, thinking it wouldn't be long but he hung in for like another 10 years.
he was old as shit when I lived with him. nobody really knew how old either. he was quiet and really gentle and friendly. he was afraid of storms so he would come find me when they were brewing and sit with his face in mine and pant heavily while rubbed his back.
his breath was absolutely horrible lol.