How old are you?

Last Friday we went to Sweet Tomatoes for a yummy lunch after an hour of playing in the park. Everyone was hungry and ready to eat. When I was paying for our meal, the cashier asked "for how many do you pay" and I said for me and my son who is 3. Right there, Lukas yelled out "no Mommy, I am 4!". How true. His 4th birthday was the day before. :) How could I have forgotten?