My dad constantly forgets how old I am (and he probably woule flat out forget it if it wasn't for the fact that my moms is three days before mine), so you aren't the only one with birthday related issues toward their offspring.
I remember my parents forgot my sister's birthday once. It was her 17th or 18th or something like that. She was so upset... But then my parents got her something really nice for her birthday but they just ended up forgetting her next one.