As Max the First neared his 17th birthday, we knew his days were numbered. He approached the wrong side of the door to go out and often started to pee in the vestibule, a common sign of cognitive impairment. Long walks — and sometimes any walks — were now out of the question.

But not until I was home all day and heard him whimper almost constantly did I realize we were not doing this much-adored pet any favors by prolonging his life. As a 40-pound springer spaniel mix wintering in New York and summering in Minnesota, he’d led a very full and active life. He hiked in the woods year-round, frolicked in every snowfall in Brooklyn’s Prospect Park and even swam a river with me summer mornings in Minnesota until he was 15.

Although he had no discernible fatal disease, we knew that as Max the First (hereafter, Max-I) passed the equivalent of 110 in people years, it was time to say a tearful goodbye. Much to our surprise, he seemed grateful. We had planned to wheel him to the vet in a little red wagon but Max-I, who until then refused to set foot on any block near the vet’s office, now walked there without hesitation under his own steam. I admit that 18 years later I still weep recalling how omniscient he seemed on his last day.

Now, though, I’m the happy parent of Max-II, a 6-year-old Havanese who still runs and swims like a puppy as he approaches what some veterinarians consider middle age for a 17-pound dog. And I hope I’m even smarter now about what it takes to keep him vibrant for as long as his biology will allow.