Kindergarten opens some doors, while others close.

My older grandchildren are about to turn 5. This is as significant an age as 13, when people trembling on the precipice of adolescence get a send-off guaranteed to test their patience. “Uh-oh! A teenager in the family!” celebrants will hear 3,000 to 5,000 times. Another significant age is 18, when a person can vote. And, of course, 21 has long been known as the age when the birthday boy or girl can order a drink at midnight.

But the first milestone age, 5, has more significance: It's the last digit on the first hand of life; the year children start kindergarten, begin learning to read and to calculate, and are expected to report to a classroom each day for hours dictated by the school and not by a parent's work schedule.

This last notion will be an adjustment for me. I'm not given to snatching my grandchildren out of day care willy-nilly. When my husband and I do pickup duty, we arrive about the same time as their parents normally do. We don't fly in on a whim at 10 a.m., feed the kids candy bars and take them back just before naptime. But, with their parents' permission, we could. What tugs at my heartstrings is knowing that when these two are kindergartners, we can't pull them out of school without an official excuse and a good reason.

Too soon, these dear hearts will be on their way through grade school, explorers with Minecraft lunchboxes and monkey-face backpacks. Kindergarten yields to first grade, second, third and school testing. Homework, after-school events, science projects, sleepovers, friendships, rivalries and disappointments are part of the deal. Schedules get so crowded when children are in school. Evenings are “a school night” or “not a school night.”

I'm not complaining about all this, mind you. Most of the footwork will be their parents' responsibility, and the kindergartners-incipient-high-school-graduates will love it all, or at any rate most of it. What might sound like complaining is in fact simple sighing. The years of pushing trucks along the floor, of being served “yummy” handfuls of playground mulch will give way to more sophisticated games. Yesterday, Eric Hill board books about Spot; tomorrow, “Silas Marner.”

My own daughters' childhoods seem, in retrospect, to have flown by, but of course they didn't. We went through what all parents go through: dazzling joy, bleak despair, breath-catching uncertainty and many, many moments of grateful regularness—days that began and ended without calamity, frustration or even minor arguments.

We collected family jokes and word mispronunciations, both of which entered the family lexicon, along with our younger daughter's sudden rendition, at age 2, of “L'Amour,” which she'd heard a chicken sing on Sesame Street. She was inspired to warble the song from her car seat one evening on our way home from dinner, and her bellowed “La MOOO!” sent first her sister and then us into paroxysms of giggles. Even now, a quiet “La moo!” has the power to make us disgrace ourselves at solemn moments.

Now our grandchildren's families are well into their own collections of beloved trivia. Even as I mourn the passing of early childhood, I see the boundless adventures that lie ahead. As the first two enter kindergarten and the third strains to follow, I'm going to suck up these sentimental regrets and enjoy it all. So help me.

Margo Bartlett and her husband have two daughters, two sons-in-law, three grandchildren and two car seats. She also writes the Just Thinking column for ThisWeek Community News. You can reach her at margo.bartlett@gmail.com.