I choose to believe that a /pol/ denizen made this to troll people until proven otherwise.

If it is diagnosed and this makes them legally recognized as legally incompetent, then yes. (Usually by diagnosing them with a mental disability)

Probably there's another way, since I heard it is possible to have someone become legally incompetent by simply spending too much (under very specific criteria), so maybe they can actually make your age-fluidness stick with some legal loophole.

"Excuse me officer currently I'm over 21"

"Sorry Xa'am, your DOB now reads age-fluid, so to err on the side of caution I have to assume you are or were at some point a minor at the wheel, so I'm taking you to child services and calling your parents."

Also, Mad Hatter and March Hare are going to have a fit trying to figure when it's your unbirthday.