这是用一行文字回答这个问题的最简单方法。

DateTime Dob = DateTime.Parse("1985-04-24");
 
int Age = DateTime.MinValue.AddDays(DateTime.Now.Subtract(Dob).TotalHours/24 - 1).Year - 1;

这也适用于闰年。

下面是一个测试片段：

DateTime bDay = new DateTime(2000, 2, 29);
DateTime now = new DateTime(2009, 2, 28);
MessageBox.Show(string.Format("Test {0} {1} {2}",
                CalculateAgeWrong1(bDay, now),      // outputs 9
                CalculateAgeWrong2(bDay, now),      // outputs 9
                CalculateAgeCorrect(bDay, now),     // outputs 8
                CalculateAgeCorrect2(bDay, now)));  // outputs 8

这里有一些方法：

public int CalculateAgeWrong1(DateTime birthDate, DateTime now)
{
    return new DateTime(now.Subtract(birthDate).Ticks).Year - 1;
}

public int CalculateAgeWrong2(DateTime birthDate, DateTime now)
{
    int age = now.Year - birthDate.Year;

    if (now < birthDate.AddYears(age))
        age--;

    return age;
}

public int CalculateAgeCorrect(DateTime birthDate, DateTime now)
{
    int age = now.Year - birthDate.Year;

    if (now.Month < birthDate.Month || (now.Month == birthDate.Month && now.Day < birthDate.Day))
        age--;

    return age;
}

public int CalculateAgeCorrect2(DateTime birthDate, DateTime now)
{
    int age = now.Year - birthDate.Year;

    // For leap years we need this
    if (birthDate > now.AddYears(-age)) 
        age--;
    // Don't use:
    // if (birthDate.AddYears(age) > now) 
    //     age--;

    return age;
}

因为闰年和所有事情，我知道的最好的方法是：

DateTime birthDate = new DateTime(2000,3,1);
int age = (int)Math.Floor((DateTime.Now - birthDate).TotalDays / 365.25D);

我的建议

int age = (int) ((DateTime.Now - bday).TotalDays/365.242199);

这一年似乎在正确的日期发生了变化。（我在107岁之前进行了现场测试。）

非常简单的答案

        DateTime dob = new DateTime(1991, 3, 4); 
        DateTime now = DateTime.Now; 
        int dobDay = dob.Day, dobMonth = dob.Month; 
        int add = -1; 
        if (dobMonth < now.Month)
        {
            add = 0;
        }
        else if (dobMonth == now.Month)
        {
            if(dobDay <= now.Day)
            {
                add = 0;
            }
            else
            {
                add = -1;
            }
        }
        else
        {
            add = -1;
        } 
        int age = now.Year - dob.Year + add;

通过较少的转换和UtcNow，这段代码可以照顾闰年2月29日出生的人：

public int GetAge(DateTime DateOfBirth)
{
    var Now = DateTime.UtcNow;
    return Now.Year - DateOfBirth.Year -
        (
            (
                Now.Month > DateOfBirth.Month ||
                (Now.Month == DateOfBirth.Month && Now.Day >= DateOfBirth.Day)
            ) ? 0 : 1
        );
}