I knew this would happen. Indeed, I told you that it would happen. My youngest son, Lindell, went into kindergarten in September as a baby, and he came out in June a little boy.

Despite knowing everything I do about little boys and the first day of kindergarten, I didn’t fully understand the ramifications of Lindell’s inevitable growth and separation from me until the summer actually started. And now I feel sad.

Here’s how a summer day went for us last year:

My older boys spent most of their day swimming in the lake, playing catch in the front yard or walking to and from the baseball field. They went to summer camp for a week and stayed all day. They played with their friends outside and left again after dinner. They knew who was in their next year’s class before I did.

Lindell, on the other hand, was still very much a baby, or a babyish toddler. He preferred sitting in my lap to swimming at the lake. He was too young to go to the baseball field alone, and he had no interest in summer camp. He waited with bated breath for me to tell him who his next year’s teacher would be and who might be in his class.

All of that has changed this summer.

Last week, all three boys went off to Windover, a hidden-in-the-woods art camp in Newburgh, Maine, that has no buildings except for old barns with slanting floors. It is total kid paradise. As young as 6, campers can help make movies, do photography, make pottery, paint, swim and run through the open fields. Lindell has never before been able to go to Windover with Ford and Owen. He’s only heard the stories and looked longingly at the art projects they brought home. This summer, he was finally old enough, and although I had my doubts about whether or not he’d actually leave me for a full day, he didn’t even look back when he climbed the steps of the bus and disappeared into it with his brothers.

So I had five full days all to myself. I could sleep in. I could shop or write or eat cookies for lunch. I had zero responsibilities. And what did I do? I sat around missing all of my boys, but in particular, Lindell.

I wandered from room to room thinking of how he usually would be on my heels asking me to play or telling me about some fantastic (and really unsafe) idea he had. I did errands and missed him sitting in the shopping cart or running up and down the aisles. I folded his clothes and nearly cried over every tiny sock.

Silly, huh? But I couldn’t shake this nagging sense of loneliness. And it is odd because Lindell just spent a whole year in kindergarten. Hadn’t I had time to adjust to all three boys being gone? Why did I not feel this way during the school year?

Then I realized the difference: all-day school was something for which Lindell had no choice. He went to kindergarten every day because he had to. Last week, Lindell went to all-day summer camp because he wanted to. That’s a whole different ballgame.

I began to understand what this year’s summer will be like: Lindell will join his brothers for swimming and leave me on the shore. He will play catch with them outside and not follow me around the house. He will go with his brothers to the baseball field and back. And in a very short time, I will have no babies left at all.

Ouch.

Late last week, I was thinking about these things when the camp bus rolled and let off the boys. Ford, 12, walked out calmly with his hands stuffed in his pockets and no expression on his face. Owen, 10, walked beside him, but he smiled at me as they came closer. But it was Lindell who broke into a sprint, his backpack bouncing against his back, to come give me a hug.

“I missed you,” he said into my shirt.

I ruffled his hair and thought, maybe I still have a little window of time left with my baby.