The letter arrived unannounced at my home, hidden amid junk mail.

It came in a plain envelope with a simple street address on the back. There was no outward hint as to its contents, and for good reason. I would have promptly thrown it away.

Yes, it was that letter. The one no one wants to receive but all one day will. The letter that makes an IRS audit seem like a lottery prize.

The one that slaps you hard on the face and tells you once and for all that you never again will fit into those 30-inch-waist jeans.

“Dear Mr. John J. Grogan,” it began.

I scanned the opening paragraph, picking out the operative phrases: “fully eligible … membership … benefits … life over 50.”

Life. Over. 50.

I began to pray. Oh, Lord, please, no. Not that. Anything but that.

For whom does the American Association of Retired Persons troll? It trolls for me.

My official AARP membership card, No. 1567627, was attached.

“Honey,” I called to my wife. “Where’s the bourbon?”

For the record, I am not 50. Not even close. Fifty remains a faraway speck on the horizon. I remain a proud member of the forty-something decade. Some of my best friends are thirty-somethings. A few are even fresh-faced twenty-somethings.

I still do ridiculously foolhardy things like teeter from a high ladder holding a chain saw.

I am not 50, OK? That’s still four months away. Not that anyone is counting.

But could the AARP wait?

The letter tried to lure me in with a long list of “benefits and services” aimed at nascent geezers-in-waiting.

A safe-driving course, for starters. I couldn’t help conjuring up a horrible premonition of me in a Buick Skylark tooling along at 43 mph in the high-speed lane of Interstate 95. Nooooo!

My membership comes with a magazine to remind me that I’m on the downhill slide to 100.

It offers regular updates on Social Security, an entitlement program I am perfectly happy to never qualify for.

I also can use my AARP card, and I quote, “to save on shoes.”

All colors, or just white?

I won’t deny it. The arrival of my AARP card threw me into a total funk. This was my parents’ organization. Why was it bugging me?

I tried to put the best spin on it. Finally, those bushy eyebrows I’ve always craved would be coming into their own!

But there was no denying the harsh reality. Life’s best chapters may still lie ahead, but any way I spun it, the story line ended the same way.

At the cemetery.

That’s when I thought about Joe Paterno. What better role model for the second half-century?

The Penn State coaching legend will turn 80 next month and still gets up every morning to go to work.

Recently he was flattened by two players who crashed into him, breaking his leg, but not his intensity.

I loved the photo of Paterno being carted off the field with his injured leg up. His face, equal parts disgust and impatience, said it all: Get the damn leg fixed so I can get back to work, will ya?

Not that I harbor fantasies of coaching college ball. But when I grow up, I want to be like Jumpin’ Joe Paterno.

Not a stubborn-as-a-mule coach. But someone who refuses to relent to the ravages of time. Someone who embraces his passion without compromise and won’t let go. Someone who tells age what it can do with itself.

A guy who won’t slow down.

My father was a bit like Paterno that way. He hurled himself at life full bore every day, right up to his last. I would have put him, at 89, up against men 20 years younger.

An automotive engineer, he liked to say, “The worst possible thing for a car is to let it sit idle.” The same rule applies to the human machine.

Bring it on, AARP. Bring on the membership cards and senior discounts.

Fifty may be right around the corner. But as a great coach might tell his players in the halftime pep talk of life: You’re only as old as you let yourself be.