The birthday kitty can have adult food whenever you get a chance to switch over. Once they are a year old, it is safe for them to start eating adult food.
Don't forget to slowly start feeding them the adult food. Mix in a little of the adult with the kitten (I use a 1/4 cup new food to 3/4 cup old food ratio when switching foods, then every 5 days or so, switch up til you are completely feeding the new food)