Today, the house is decorated in hibiscus flowers, fake grass and leighs. Walk up our front walk and you’ll be welcomed by an army of lit tiki torches standing at attention. Walk in and you’ll smell pork and pineapple (but she drew the line at SPAM). And after this day, our lives are changed forever. Because on this day, our little perfect Gabrielle becomes an adult.

In the life of a child, all birthdays give you reason to pause and reflect on what’s passed and how fast it’s happened. But I’ll tell you, birthday No. 18 is a big one to wrap one’s mind around.

Thinking on it, when I started writing this column, Gabrielle was only 5. She loved Polly Pockets with all her heart. Bath time involved chasing her with a rubber ducky that advanced underwater as William hummed the theme music of “Jaws.” She needed help tying her shoes.

Now, she loves records and clothes from Goodwill. Baths are elaborate affairs with gold bath bombs, charcoal soap, coconut conditioning hair treatments, and listening to podcasts until we scream at her from the main floor that it’s time for dinner. And often, she elects not to tie up her Doc Martens.

In short, I can’t believe how fast it’s gone. And I can’t believe how lucky we’ve been to have a kid like Gabrielle.

Of course, for me as stepmum, she’s the greatest gift William ever gave me. Years ago, my Tante Denise told me that would be the case (as it was with her stepchildren) and she was right. Without this kid, and without her generous, kind mom Lisa, who always let me be involved, I would have never known the satisfaction, greatness and complexity of helping to parent a child.

But beyond that, we’ve been so lucky to have this kid, this particular one. I’ve never written at length about her personality, but, since she’s an adult now, it’s certainly time.

Apart from her dad and maybe our friend Bjorn, Gabrielle is possibly the smartest person I know, in possession of a quiet, constant intelligence that pops up in the way she does just about everything every day. In the mornings, after I pack her lunch, she will gently rearrange the contents so they fit in the kit perfectly, saying nothing as she does so. When we’re playing cribbage, she never rushes me when I’m tallying my score, though she’s figured it out almost instantly. She assembles furniture and plays videogames almost as if there is nothing to navigate. On her second try, she got a 34 on the ACT. The highest you can get is 36.

But while wildly bright, Gabrielle is, above all, kind. So very, very much kinder than I was at that age. Even when faced with my deficits, she just helps, rather than judges. She bakes (great) cookies for her friends at exam time. The first question out of her mouth when she gets home after school is, “How was your day?”

She’s also always been socially fluid. She’s capable of going into any room, figuring out who she wants to be friends with, and getting to work to make that happen. And she picks the right kinds of friends, the kinds that are quirky and authentic, the kinds that love to laugh but are funny themselves — just like her.

While Gabrielle definitely has a nerdy, grungy side, she’s retained the girliness that, as a little girl, made her want to wear tiaras everywhere she went. In turn, she has become unusually good at applying makeup, something I think will likely be a signature of her generation of girls. Gabrielle is extremely striking anyway, but sometimes when she comes down those stairs in the morning, she looks straight out of a magazine. It’s lucky for me, because before I go out for dinner or to a party, Gabrielle will invariably do my makeup and give me advice on what to wear and how to accessorize. She loves to do it, her natural perkiness shining through even when she’s having a blah day.

And, finally, Gabrielle has the ability to be laidback and roll with the punches. William and Lisa have always split custody, which resulted in Gabrielle constantly going back and forth between two households, ours being the strict, “let’s listen to NPR” one, her mom’s being the fun, Disney World one. But Gabrielle has never complained, she’s just adapted in a hundred different ways.

So, as you can see, she’s a pretty special girl who certainly deserves her special Aloha Friday-themed birthday party. Soon, Lisa will be here with a deluxe cooler and a drink maker to make fruity mocktails. We pick the cake up in a few hours. And by 5, the house should be filled with special friends who’ve helped form the adult version of Gabrielle.

So, goodbye, our little perfect girl. May I be the first to say hello to the beautiful, thoughtful woman you’ve become.