One of the mysteries of the human mind is why we become angrier as we age, frequently going from high-spirited youth to mellow middle age to peed-off senior in an arc of only five or six decades.

You’d think oldsters would be happy. They bought everything they wanted or needed ages ago. The house is paid for, or cashed in for a long-desired smelly condo or rusty RV. Their kids are gone or in any case don’t call them, their parents are dead, their friends are dead, even their pets are dead — what’s not to enjoy? With so many of life’s problems solved and nuisances done away with, why aren’t these oldsters radiating happiness like wrinkled little nuclear reactors?

I think I know the answer. The conundrum that ticks off most bags and geezers is that they’re not dead yet.

That they still have to go through death aggravates them. They’ve already gone through everything life can throw at them — marriage, divorce, rising gas prices, overpriced cruises, quantitative easing, preemptive war, purchasing an unnecessary home security system, loss of hair and bone mass and memory, a traitor in the White House again — and still it isn’t over. They still have to die, and they can’t get over that.

I’ve seen this ravaging of the brain first hand. When I was a child, I had a lovely shriveled grandfather of about 70 who seemed to adore his retirement outside the mines. Aside from of a nagging, chronic cough and an addiction to coal dust, his life appeared idyllic. He had a large vegetable garden, and whenever I visited he’d take me out in the yard to show me where a mole was digging underneath and eating his produce’s roots.

“See his hole?” he’d say as he pointed out the animal’s tiny tunnel. The two of us were delighted in nature’s way. After thirty minutes of finding the hole here and there in the garden and Grandpa saying “see his hole?” each time (obsessive compulsion was part of his pleasure), we’d go inside where he’d sit before his potbelly stove chewing tobacco and spitting most of it into a tin can, and the rest down the front of his wooly vest. I’d sit beside him on a slack sofa that smelled of Mail Pouch and read a Classics Comic from the 1950s. Sometimes, when he moved his head the right way, I’d stare with affection deep inside his hairy nose.

Maybe grandma would sit beside us, and what a friendly old woman she was. She liked nothing better than to send me down to her dank, cobwebby root cellar for a jar of pickle relish, then throw a fit and whip me when I returned with beets instead, as I usually did since I couldn’t tell the difference in the dark.

Caught up in her game, if that’s what it was, she would grow red in the face and order me outside to fetch a “hickory-t,” a hickory branch off the tree in the front yard that she would lash me with for punishment, though I never received a single blow due to her poor aim and hysterical laughter.

What a cheerful, carefree couple! What fun we had when I visited them, though I’m glad Granny never got me with that “hickory-t.” I was a delicate child, and a whipping with such an unsanitary implement might have given me eczema.

Press ahead ten years. I’m a teen now, and my grandfather throws his mail-order teeth on the dinner table and growls at everyone, drooling. He smells more and more like a spittoon and won’t bathe. Has the mole’s hole, still visible in the garden, caused his mind to snap?

And Grandma is so irritated by having to cook for her grandchildren, even though she jarred all the greasy beans and acidic jam she could ever need 50 years ago and the glass containers line her root cellar, she can’t stop grousing. Every time she catches sight of me, a tender youth showcasing dental braces and his first pimples, she brands me a draft resister and a socialist agitator. Whoa, Grandma and Grandpa! What the hell happened to your easy demeanor?

The worst part is, both my grandparents received Medicare and excellent medical treatment, and at age 80 had ten or more years of constant anger to look forward to before they could finally croak and relocate to the family gravesite down by the creek. My grandfather’s mood in his last decade varied from rage to sullenness, and not once did he crack a smile to reveal his crooked Sears & Roebuck teeth. My grandmother got to the point where she could scarcely bear another day of perfect health and uninterrupted leisure and died a joyless, broken woman at 96.

And it isn’t just my grandparents. My wife recently put her mother in a retirement center (read: pre-graveyard) so that Myrtle could receive the around-the-clock watering and pruning that she requires. There she sits on the second of four floors of similar bags and geezers who are either boiling over with rage or comatose on medications. There’s no in-between state.

On an early visit I tried to embrace the old babe in a heartfelt hug, and she raked the lengths of my uncovered forearms with untrimmed, yellow fingernails (how did they grow so long in a week?), requiring me to apply antiseptic. And that was on a good day, when she was relatively calm and not fastened to the sides of her bed in a straitjacket.

Ah, peaceful old age. And don’t think for a moment the oldsters enjoy all those medications they’re forced to take. It’s only young people who like drugs. In fact, they love them. But your typical home resident is just doddering out on them. Nor is their zest for life enlivened by any activity no matter how festive. From a Sunday enema to balloon volleyball, it’s all a drag. More than a drag, it infuriates them.

I think I know how this transformation takes place. At around age 75, the riddle of death takes hold of a person’s mind. The sheer insolubility of the enigma affects you like sticking your hand in a running blender, realizing you’ve lost all your fingertips, but for a split second there’s no pain, only dumb wonder.

That split second is the time you have to figure out death. It grows into years without any increase in your understanding and with the same sense of dread. You keep waiting for the answer like you wait for the pain in your fingers.

As the moment expands, it dawns on you that there is no answer, not a hint, not ever, for you or anyone. At last you’re going to have to pay for those lost fingertips. The pain is coming, and there’s nothing you can do.

That’s when you get good and teed off, and you stay that way for a decade or so.

Then it hits you.