DateUtilities.cs

public static class DateUtilities
{
    public static int CalculateAge(DateTime birthdate)
    {
        var age = DateTime.Today.Year - birthdate.Year;
        var temp = new DateTime(DateTime.Today.Year,
                    birthdate.Month, birthdate.Day);

        if (temp > DateTime.Today)
        {
            age--;
        }

        return age;
    }
}