My daughter started high school last week.

We took her to orientation, which she entered with a face full of boredom and sighs of long suffering. That’s pretty much the way she goes everywhere these days. At least, everywhere she is accompanied by her mother and me.

Needless to say, she is 14.

So anyway, halfway through the half-day program, I spotted a poster in a hallway. “Thirty years from now,” it said, “it won’t matter what shoes you wore, how your hair looked, or the jeans you bought. What will matter is what you learned and how you used it.”

I don’t know if those words resonate with the kids who see that poster every day, but they did with me. As it happens, this was three days after my 30-year high school reunion.

My daughter was in another part of the campus. Otherwise, I’d have draped an arm over her shoulder, read those words aloud and explained their profound truth.

At your age, I would have said, most of the things you worry about are of the moment. Your hair, your shoes, your jeans. Plan for the long term, I would have said. See the big picture, I would have said. Invest your greatest energy in things that will still matter even six months down the line, I would have said.

And she would have gazed at me with awe and thanked me for sharing sage wisdom.

No, I’m kidding. Actually, she would have shifted her weight to one leg, folded her arms and rolled her eyes to the ceiling until I was finished.

I probably would have done the same thing had my mother given a similar speech back when I was 14 years old and still knew absolutely everything there was to know.

Those days are long gone. If I didn’t know that before, I sure knew it sitting there at my reunion watching all those middle-aged people search middle-aged memories, squinting at one another’s name tags before exchanging uncertain hugs and even less certain questions.

“Don’t you remember …?”

“Didn’t you used to …?”

“Aren’t you the one who …?”

It struck me that, once upon a time, the good opinion of these people was the most important thing in my life. They were the mirror of me in a thousand ways large and small, the ones who let me know when I fell short in matters of dress, deportment and general cool. They were the barometer of my social acceptance or lack thereof. I so desperately wanted to be approved by them.

And now I couldn’t remember most of their names.

One guy hugged me when he saw me. Then he hugged me as I stood in line waiting for my souvenir T-shirt. Hugged me again as I made my way back to my seat. I had no idea who he was, a fact I kept trying to impress upon him without success. Apparently, we were real tight back in the day.

I thought of that the next week as my daughter crossed her new campus, shrieking and giggling in the company of some other girls. I wished I could capture the moment and hold it for her.

Or even explain it to her. Of course, I can do neither.

Teenagers are creatures of immediacy. For them, last year is distant history and next year a million years away and nothing matters except what is in front of you now.

But if you manage to string enough heartbeats together, you gain a different perspective. Travel through enough passages, and you learn the truth of the cliche: Life’s only constant is change.

And one night you find yourself in a room full of strangers who used to be classmates, wondering how they got so old while you stayed the same. But you remember, too, that once upon a time, back when nights were young and music never quite loud enough, they were the most important people in your world.

There’s no way my daughter would get that if I tried to explain. But I have no doubt she’ll understand someday, 30 years from now.

Leonard Pitts Jr., winner of the 2004 Pulitzer Prize for commentary, is a columnist for the Miami Herald.