Who's birthday is it?

This past week, Allison's class had a few birthdays. She came home Wednesday with a handmade birthday card, insisting that it was her birthday (so much so that the bus driver even sang "Happy Birthday" to her on the way home). It took a little talking from us and her teacher to help her realize that she already had her birthday. Today, once again, she still insisted that it was her birthday. Matthew, on the other hand, has this theory that whenever anyone has a birthday, he also gets "older".

This evening, we said our good nights to the kids and asked them about what special thing was happening tomorrow. "Mommy's birthday", they responded. Matthew continues, "And you are going to be nine tomorrow." "Yes, Matthew, I'm going to be nine tomorrow." (All year long they've been saying that I'm only eight; but eight is still much older than they are.)

"And how old are you going to be tomorrow, Matthew?"
"I'm going to be four."
"No, Matthew. Your age changes only on your birthday and that isn't until August. How old will you be tomorrow?"
"I'm going to be three."
"That's right."

Now for Allison.
"How old is Mommy going to be?" "Nine."
"And how old are you going to be?" "I'm going to be nine too."
"No, sweetie, your birthday is in January. You won't get older until then. Remember, your birthday came after Christmas."
"Why is my birthday in January?"
"Because that is when you were born."
"Why." "Good night, Allison."