My son plans to go to his first New Year's Eve party since he turned 18. Here's a letter I wrote to explain why I'm such an obnoxious parents. Dear 18-Year-Old Brain, Thanks so much for managing my son for all these years. What a time you two are having together! I remember when you learned where your toes were, how to count, and what hot and cold felt like. I'll never forget the day you solved a complex math problem that I could not. Believe it or not, it delighted me to realize your development was surpassing my own in that way. A few weeks ago, we celebrated your 18th birthday. Congratulations! I am so proud of who you are and of all you've learned. You are a senior in high school, and so many blessed opportunities await you. Funny thing, though: you sort of thought passing that 18-year-old mark means you're all grown up. In many ways, that's true. You can now vote, join the armed services, get married, buy firearms, sue in your own name, and...