The age for the COPPA is checked once at the registration process. Once the age is ok, even informing a wrong birth date, you won’t be asked again to fill it, that’s why it doesn’t store. The user can even change their data, including birthday. I don’t remember any control on it (at code level) but I actually never really use it so I’m not sure.
You didn’t say in your post above what do you really want. What’s the goal? Check date for what? When?