人们可以这样计算“年龄”（即“西方人”的方式）：

public static int AgeInYears(this System.DateTime source, System.DateTime target)
  => target.Year - source.Year is int age && age > 0 && source.AddYears(age) > target ? age - 1 : age < 0 && source.AddYears(age) < target ? age + 1 : age;

如果时间方向为“负”，则年龄也将为负。

可以添加一个分数，代表从目标到下一个生日的累计年龄：

public static double AgeInTotalYears(this System.DateTime source, System.DateTime target)
{
  var sign = (source <= target ? 1 : -1);

  var ageInYears = AgeInYears(source, target); // The method above.

  var last = source.AddYears(ageInYears);
  var next = source.AddYears(ageInYears + sign);

  var fractionalAge = (double)(target - last).Ticks / (double)(next - last).Ticks * sign;

  return ageInYears + fractionalAge;
}

分数是过去的时间（从上一个生日到下一个生日）与总时间的比率。

无论是向前还是向后，这两种方法都以相同的方式工作。

private int GetAge(int _year, int _month, int _day
{
    DateTime yourBirthDate= new DateTime(_year, _month, _day);

    DateTime todaysDateTime = DateTime.Today;
    int noOfYears = todaysDateTime.Year - yourBirthDate.Year;

    if (DateTime.Now.Month < yourBirthDate.Month ||
        (DateTime.Now.Month == yourBirthDate.Month && DateTime.Now.Day < yourBirthDate.Day))
    {
        noOfYears--;
    }

    return  noOfYears;
}

这是一个非常简单的方法：

int Age = DateTime.Today.Year - new DateTime(2000, 1, 1).Year;

要使用最近的年龄计算年龄：

var ts = DateTime.Now - new DateTime(1988, 3, 19);
var age = Math.Round(ts.Days / 365.0);

MSDN帮助为什么没有告诉您这一点？看起来很明显：

System.DateTime birthTime = AskTheUser(myUser); // :-)
System.DateTime now = System.DateTime.Now;
System.TimeSpan age = now - birthTime; // As simple as that
double ageInDays = age.TotalDays; // Will you convert to whatever you want yourself?

我已经创建了一个SQL Server用户定义函数来计算某人的年龄，给定他们的出生日期。当您需要它作为查询的一部分时，这很有用：

using System;
using System.Data;
using System.Data.Sql;
using System.Data.SqlClient;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;

public partial class UserDefinedFunctions
{
    [SqlFunction(DataAccess = DataAccessKind.Read)]
    public static SqlInt32 CalculateAge(string strBirthDate)
    {
        DateTime dtBirthDate = new DateTime();
        dtBirthDate = Convert.ToDateTime(strBirthDate);
        DateTime dtToday = DateTime.Now;

        // get the difference in years
        int years = dtToday.Year - dtBirthDate.Year;

        // subtract another year if we're before the
        // birth day in the current year
        if (dtToday.Month < dtBirthDate.Month || (dtToday.Month == dtBirthDate.Month && dtToday.Day < dtBirthDate.Day))
            years=years-1;

        int intCustomerAge = years;
        return intCustomerAge;
    }
};