只是因为我认为最重要的答案不是那么明确：

public static int GetAgeByLoop(DateTime birthday)
{
    var age = -1;

    for (var date = birthday; date < DateTime.Today; date = date.AddYears(1))
    {
        age++;
    }

    return age;
}

这里有一个DateTime扩展程序，它将年龄计算添加到DateTime对象。

public static class AgeExtender
{
    public static int GetAge(this DateTime dt)
    {
        int d = int.Parse(dt.ToString("yyyyMMdd"));
        int t = int.Parse(DateTime.Today.ToString("yyyyMMdd"));
        return (t-d)/10000;
    }
}

var startDate = new DateTime(2015, 04, 05);//your start date
var endDate = DateTime.Now;
var years = 0;
while(startDate < endDate) 
{
     startDate = startDate.AddYears(1);
     if(startDate < endDate) 
     {
         years++;
     }
}

这是一个非常适合我的功能。没有计算，非常简单。

    public static string ToAge(this DateTime dob, DateTime? toDate = null)
    {
        if (!toDate.HasValue)
            toDate = DateTime.Now;
        var now = toDate.Value;

        if (now.CompareTo(dob) < 0)
            return "Future date";

        int years = now.Year - dob.Year;
        int months = now.Month - dob.Month;
        int days = now.Day - dob.Day;

        if (days < 0)
        {
            months--;
            days = DateTime.DaysInMonth(dob.Year, dob.Month) - dob.Day + now.Day;
        }

        if (months < 0)
        {
            years--;
            months = 12 + months;
        }


        return string.Format("{0} year(s), {1} month(s), {2} days(s)",
            years,
            months,
            days);
    }

这里是一个单元测试：

    [Test]
    public void ToAgeTests()
    {
        var date = new DateTime(2000, 1, 1);
        Assert.AreEqual("0 year(s), 0 month(s), 1 days(s)", new DateTime(1999, 12, 31).ToAge(date));
        Assert.AreEqual("0 year(s), 0 month(s), 0 days(s)", new DateTime(2000, 1, 1).ToAge(date));
        Assert.AreEqual("1 year(s), 0 month(s), 0 days(s)", new DateTime(1999, 1, 1).ToAge(date));
        Assert.AreEqual("0 year(s), 11 month(s), 0 days(s)", new DateTime(1999, 2, 1).ToAge(date));
        Assert.AreEqual("0 year(s), 10 month(s), 25 days(s)", new DateTime(1999, 2, 4).ToAge(date));
        Assert.AreEqual("0 year(s), 10 month(s), 1 days(s)", new DateTime(1999, 2, 28).ToAge(date));

        date = new DateTime(2000, 2, 15);
        Assert.AreEqual("0 year(s), 0 month(s), 28 days(s)", new DateTime(2000, 1, 18).ToAge(date));
    }

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);

另一个功能，不是我做的，而是在网上找到的，并做了一些改进：

public static int GetAge(DateTime birthDate)
{
    DateTime n = DateTime.Now; // To avoid a race condition around midnight
    int age = n.Year - birthDate.Year;

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

    return age;
}

我只想到了两件事：来自不使用公历的国家的人呢？DateTime。我认为现在是服务器特定的文化。我对实际使用亚洲日历一无所知，我不知道是否有一种简单的方法来转换日历之间的日期，但以防万一，你想知道4660年的中国人：-）