Birthday on Leap Year

On febuary 29 people could say they only have a birthday every four years. If i had a birthday on febuary 29 i would definetly say i only had a birthday every four years and say if i was 12 i would say im only three. However if you used this tactick you couldnt buy products or other things like it until you are past forty years old. So i wonder how that works for some of the people who do this.