On Facebook, a friend has posted “Here’s to chapter 9” on her DD’s 9th birthday. I keep seeing this - e.g. recently “here’s to chapter 27!” on a 27th birthday.
They’re all wrong. Chapter 1 starts when you’re born. Chapter 2 is 1-2. So Chapter 10 starts on your 9th birthday and chapter 28 starts on your 27th.
I’m not saying anything on Facebook, of course, but come on! It’s obvious, isn’t it?!