you cant change ur nnid or pnid birth date

idk unc

From what I can tell you can change the birthday date in the User Settings on the Wii U. I don't know about the 3DS since I've never owned one.

On 3DS, if you made your Pretendo account from the console(not the website) you should be able to change it just fine, otherwise idk how to help you further :(