And suddenly your oldest child is an adult by almost any legal definition...

Tomorrow, my daughter Elissa, the person through whom I learned to change a diaper, make a ponytail, and tolerate hours of "Winnie the Pooh" videos, turns 21 years old.

I won't get into how mind-blowing this is, because a billion people before me have tried – with varying degrees of success – to express what it means when a person you raised from fetus-hood to some semblance of young adulthood is suddenly on the verge of becoming an honest-to-goodness, unequivocal grown-up.

But I will say that I'm having a hard time getting my arms around it. It's not that I don't accept it (it's actually kind of cool). It's just that it doesn't seem possible.

You know that someday your child is going to grow up. They will grow older and maybe get married and have kids of their own. But in those early years when you're exhausted just caring for them and keeping them alive, the concept is misty and theoretical. Your brain knows it's going to happen, but your heart doesn't.

Yet here we are, just 24 hours before she can go down to the store and buy a bottle of Mad Dog and drink herself silly until she vomits in a legitimate, legally compliant manner. (NOTE: I will assume that is not how she'll spend her birthday.)

And the whole cliche about time going way too fast becomes real and breathtaking and maybe a little bit painful, too.

I don't see Elissa a lot these days. She lives 20 minutes away on her college campus, and our lives are both filled with commitments and people and long hours of work. I love when I come home and she's there, though, and I really love when it's winter break and she's with us for an entire month. But those times are getting fewer and further between.

I don't post about her on Facebook as much as I used to because she's not doing the things her four younger siblings are doing that are easy to brag about: playing soccer or winning an award at school or playing a solo on her oboe (she used to play oboe...I have the credit card receipts from years of private lessons to prove it).

Instead, she's doing what adults do. She's studying and planning and beginning the process of carving out a niche for herself in the world. She's doing exactly what we as parents want for our children: She's proving that we prepared her well enough to launch herself into the world with some degree of competence and understanding.

And while she doubts herself constantly, I can tell you she's doing it beautifully. I'm so proud of her it hurts. I'm so proud of her I can't type this sentence without getting misty-eyed. I'm so proud of her and what she's becoming.

And I guess that's my job now. I give advice when asked. I help when I can. And I stand back and beam with pride while Elissa goes about the business of being whatever Elissa is going to turn out to be.

It's a role I always knew I would have to fill. I just didn't realize it would come so soon.