Just like that, my oldest son decided I was no longer “Daddy.”

I would now be known as “Dad,” and I would be addressed in a slightly detached, very first-grade and decidedly un-baby way. Jack, you see, has “joined up with a group at school,” as he puts it, and he has learned quickly that you just don’t refer to your old man as “my Daddy” unless you want to be called a baby. In the world of first-graders, few things are more disastrous.

I acted like it was no big deal. “I understand,” I said, my back to him. “You gotta do what you gotta do.”

“That’s right,” he huffed. “I’m not a baby.”

I turned, knelt in front of him and whispered. “But I just want you to know … You can still call me Daddy at home, when no one’s listening.”

Jack shook his head. “No, Dad. You’re not my daddy anymore. You’re my dad.”

A pang hit me in the gut. I bit my lip and looked down. “I know, honey. But you know, if you’re just feeling bad one day, and maybe we’re hugging, and you want to call me Daddy when no one’s around, that’s perfectly fine with me. You know, when you’re feeling sad inside or something. I’m just saying.”

Jack considered this. “Maybe.”

Afterward, I found myself asking, What the hell happened? How did we get here so quickly? What happened to those blurry days, those new-parent days when the skies seemed darker, the streets emptier than they really were, when our world was so small and simple and reduced, when I could barely keep my eyes open and this little bald kid would look up at me with the biggest smile and belt out all these goo-goo’s and ga-ga’s, when he’d waddle toward me with this open mouth, his arms outstretched, when I was his Daddy and he didn’t care who the hell knew?

Well, first-grade happened.

It happened quick. I can tell you that at the beginning of first-grade, he was still referring to me as “my Daddy” and throwing out the “Hey, Daddy’s” dozens of times a day. And then one day, maybe three weeks ago, bam — the “Daddy’s” stopped. And damn it if the kid is not sticking to his word, consistently calling me “Dad” and nothing else.

“Hey, Dad, check this out.” … “Dad, did you know that birds are direct descendants of dinosaurs?” … “Please, Dad. Just five more minutes?” … “Dad, Dylan won’t stop following me.”

But I still have Dylan.

“You can call me Daddy as much as you want, okay, kiddo?”

“Yep.” Our 3-year-old beams at me. “You’re my Daddy.”

“Yes, I am,” I announce to no one in particular.

Dylan might still call me Daddy, but there are signs (good healthy signs, I might add) that he, too, is ushering us out of the cute-and-innocent era.

Just this week, Dylan completed the long process of becoming completely potty-trained. Now don’t get me wrong. I’m thrilled. My wife is thrilled. We’ve been changing diapers for six years and four months, and oh, what these eyes have seen. I will not miss soggy diapers at the park, nor that tell-tale look on Dylan’s face at the restaurant when the food is getting served, that strained expression that told us we had a deuce drill in our immediate future.

So yes, I understand how much easier life will be, and I’m thrilled. And I understand that it’s a good thing when an individual no longer prefers to crap in his pants. But I also know that we’re bidding farewell to a time that will never return, and I guess I’m kind of a nostalgic guy sometimes. Because I have a son who’s already six, a son who seemed to be two not that long ago, I can see now how quickly all of this will happen, and how inflexible the passage of time will be. There ain’t no going back — the kids won’t let you.

I’m thrilled about the future. Each month just gets more exciting, and challenging. So I am looking forward. But I guess I’d still pay some big money for one last time, one last time to see my oldest son run across the playground when he sees his father — yelling, “Daddy … Daddy … Daddy.”