In C#, how do I calculate someone's age based on a DateTime type birthday?

Question

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

Answer 1

I've made one small change to Mark Soen's answer: I've rewriten the third line so that the expression can be parsed a bit more easily.

public int AgeInYears(DateTime bday)
{
    DateTime now = DateTime.Today;
    int age = now.Year - bday.Year;            
    if (bday.AddYears(age) > now) 
        age--;
    return age;
}

I've also made it into a function for the sake of clarity.