Sorry, but I don't see it this way. It's one thing if you happen to be at WDW on your actual birthday, but if you are celebrating weeks or months away from that actual date, you are not celebrating your birthday -- you are celebrating the mere fact that you have a birthday, which makes you exactly the same as everyone else in the park. How does that validate wearing a tiara?
If you want to wear a tiara to celebrate being at WDW, I say go for it. But pretending you are having a "special" day so people will treat you in a special way is, in my view, a violation of the social contract. Even when kids do it with the explicit permission and cooperation of their parents. Even an actual birthday isn't really all that special, but at least it is relatively rare (once per year). If every day is everyone's birthday, who cares?
I think the entire WDW experience would be better for everyone if there were fewer people there working hard to convince themselves they are more special than the people around them.