Last month, while walking the dog on the morning of my 50th birthday, I remembered that only one of us knew about time’s increasing claim on our lives.

I couldn’t have ignored the approach of my half-century mark if I’d tried. In the weeks before the big day, friends and loved ones reminded me that I’d soon be five decades old.

Birthdays pass more casually for our terrier, Foster, since he’s a former stray, and the exact date of his birth is a mystery. He gets gourmet dog food on the anniversary of his arrival at our house, and we know that he’s at least a dozen years old, which ranks him, among his kind, as a senior citizen.

But the news of his advanced maturity hasn’t reached him yet. Foster never looks in the mirror, never sees the growing gray patch around his snout. He seeks out spots of sun on the driveway or dining room floor, warming his old bones by sitting, sphinx-like, in the little circle of light. Each morning, he stretches himself to life on the porch stoop, a few moments of canine calisthenics to get limber for his daily stroll.

Beyond these small concessions to age, Foster continues to think like a puppy. He chases tennis balls and nabs them in his jaws, gripping them so fiercely that he must sometimes, foolishly, seek his master’s help in freeing his fuzzy prey from his mouth.

Although he walks the same path through the same neighborhood, each day seems like a new book to him.

Time moves slowly for a terrier, since they tend to see each moment frame by exacting frame, as if holding a spool of film before the lamp.

Foster glides glacially down the street, sniffing every shrub and tree, reading every clause of creation, looking for the fine print within earth and grass that might reveal to him the rich secrets of the morning.

Midway through my birthday walk, we stopped in front of an office building with a black ribbon bow across its door, a small gesture of mourning for Sherry Scardina, an education administrator who passed away suddenly last month while doing the work she loved.

Sherry was a great help to my son when he needed a guiding hand, and her death was a vivid reminder that on any birthday, the best gift isn’t a tie or a wallet or a fishing rod, but life itself, the chance to rest closely within the embrace of those who love us.

Back at home, Foster enjoyed a dog biscuit before heading to his bed for a midmorning nap.

He orders his life according to a few simple principles:

  • Begin each day with a stretch.
  • Exercise.
  • Play hard.
  • Get plenty of rest.
  • Stay curious.
  • Seek out the sun.
  • Have a treat at least once daily.
  • Love often, and without condition.

If there are better rules for navigating life after 50, I haven’t found them yet.