I guarantee you that you didn’t accidentally add the wrong birthdate. I’m sure you added it correctly the first time. It’s an issue with the app. I know this because I added 5 people and all of their birthdates were wrong (all were saved as one day early). I caught it in time and the only way to fix it was to list their birthdates as one day later, which tricked the app into saving the correct date.
Unfortunately I had to cancel my old tickets and purchase new ones (because there’s no simple way to exhange anything on this site). And this time I didn’t catch it in time. So now -- once again -- all five birthdates are incorrect.
Just do a search in these forums for the word “birthdate” and you will see that you’re far from alone! But rather than identifying the problem and fixing the app, you have to go through the arduous process of walking someone in customer service through changing it for you, and that’s only if you haven’t already activated your pass. (If you did that you’re going to end up having to refund and repurchase because come on, why make anything easy around here?)