How do I calculate someone's age based on a DateTime type birthday?

Given a DateTime representing a person's birthday, how do I calculate their age in years?

An easy to understand and simple solution.
// Save today's date.
var today = DateTime.Today;

// Calculate the age.
var age = today.Year - birthdate.Year;

// If the birthdate hasn't arrived yet, subtract one 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.

Stack Overflow Q#9, score: 2433