// Save today's date.
var today = DateTime.Today;
// Calculate the age.
var age = today.Year - birthdate.Year;
// Go back to the year in which the person was born in case of a leap year
if (birthdate.Date > today.AddYears(-age)) age--;
However, this assumes you are looking for the western idea of the age and not using East Asian reckoning.
Best Answer
Jeff, your code is nice but could be clearer with constants (as suggested in Code Complete).