Given a DateTime representing a person's birthday, you can use following code snippet to calculate the age.
Let's suppose, your date of birth is 18 December 1980
DateTime birthday = new DateTime(1980, 12, 18);
Now you can calculate your age using following code
DateTime today = DateTime.Today;
int age = today.Year - birthday.Year;
if (birthday > today.AddYears(-age))
{
age--;
}