Happy Birthday to J. K. Rowling, born July 31, 1965.
So, why do we celebrate birthdays? And what's the deal with the law of birthdays? Well, of course, the date of one's birth matters for things such as when one is allowed to drive, and get married, and disavow contracts, and be tried as an adult, and BTW start school. If you're not automatically a certain age, I understand, you can't start kindergarten or first grade--you have to wait under the next year--or be certified by the school psychologist as "mature enough" to enter with the cohort roughly the same age as you. Hence, the sitcom episodes about "being held back" and "being nearly a year older than the rest of the kids in the class." This is rough when one is young and can't go to school with one's friends. It used to be done routinely when I was in elementary school. I gather it is not done so routinely now.
Once young people turn 18, they're adults, and their grades, medical records and in some cases insurance become their concern.
Ah, the magic of being a certain age--on a particular day and year. On the day before, you are not legally permitted to drive or drink or go to war and kill (legally) because your country wants you to do so. And on that day, you are.
Here's a post from the Volokh Conspiracy on the law of birthdays.