Answer:
DateTime now = DateTime.Today;
int age = now.Year - bday.Year;
if (now < bday.AddYears(age))
age--;
So your helper method would look like:
public static string Age(this HtmlHelper helper, DateTime birthday)
{
DateTime now = DateTime.Today;
int age = now.Year - birthday.Year;
if (now < birthday.AddYears(age))
age--;