Washington Post columnist John Kelly’s dog Charlie, resting in Silver Spring, Md. on Sept. 9, 2012. (John Kelly/The Washington Post)

Time is a funny thing. Einstein said that if you launched one twin in a spaceship and kept the other on Earth, the space twin would hardly have aged at all when he returned years later while his brother would be old and gray.

Of course, you don’t need a spaceship to witness time’s elasticity. Just get a dog. Mine used to be younger than I am, and now he’s older than I am. That’s a good thing, actually, for there are worse ways to learn about aging than by watching a dog.

When we met, Charlie was 18 months old. He was loose-limbed and antic, barely able to control himself. Labrador retrievers keep their goofy, coltish demeanor until they’re 3 or 4, and it wasn’t till then that he started settling down.

I was about to turn 40 when we got Charlie. That was 10 years ago. In that time, Charlie has shot past me, agewise. He’s now the senior partner, white of muzzle and with a creakiness in his joints. I look at him and think, “How did you get so old?”

But let’s not kid anybody. I’m aging, too. Maybe not as fast as Charlie, but with just as much inevitability.

My 50th birthday is coming up and despite the AARP bromides — you’re only as old as you feel; age is just a number; 50 is the new 40 — I’m not exactly looking forward to the milestone.

There’s been a subtle but definite shift in the balance of power between my brain and my body — that is, between the me that I think I am and the me that I really am.

My brain used to tell my body what to do. With barely a thought, my brain would issue various commands and my body would instantly, uncomplainingly, execute them. Oh, an eager little beaver was my body.

Now the tables are turned. After years of kowtowing to that gray lump of neurons in my skull, my body realizes it’s in the driver’s seat at last.

Out goes the message from my brain: Squat on the floor to access the trap under the kitchen sink.

Back comes a message from my body: “You want me to do what?”

Squat. Sink. Trap.

“Yeah. Sorry. Not gonna work.”

My body sounds like a slightly peeved general contractor explaining why the cabinet I ordered can’t be installed the way I wanted.

“Oh, I can squat on the floor,” my body says. “I can squat all day. But it’s going to cost you. The knees aren’t what they used to be. Ankles are a little shaky, too. Tell you what: I’ll squat, but I’ll need to grab the countertop to haul myself upright.”

My brain decides to call a plumber.

Do I sound like a wreck? I’m not, really. But it doesn’t take an actuary to see that I’m on the downhill slide, just as Charlie is.

When I saw that he’d started to get tufts of gray fur between the pads of his feet, I realized it wouldn’t be long before I got whatever the human equivalent is. When I see the careful way he lowers himself to the ground — his hips stiff as he settles into what I’m sure he sees as a dignified collapse — I joke, “You look like an old man, Charlie,” knowing full well that I will be that old man.

I don’t know how Charlie feels about getting older. I like to think he’s accepting of it, just as I hope to be. In fact, I hope he’s enjoying it.

Just now, he is dozing behind me, the curve of his butt touching a wheel of my swivel chair. Every now and then he stretches his legs out, cat-like, knits his claws into the carpet and does a languorous scriiitch. Then he resettles himself and falls back asleep.

One front paw is crossed over the other. It twitches slightly, like a canine benediction.

Aging is unknown territory for each of us, despite the fact that humans have been doing it forever. I think there are worse ways to spend your final years than napping next to someone you love, dreaming of what was and what still might be.

To read previous columns, go to washingtonpost.com/johnkelly.