TimeSpan diff = DateTime.Now - birthdayDateTime;
string age = String.Format("{0:%y} years, {0:%M} months, {0:%d}, days old", diff);

我不知道你到底希望它返回给你多少，所以我只是做了一个可读的字符串。

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。

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;

        var EndDate = new DateTime(2022, 4, 21);

        var StartDate = new DateTime(1986, 4, 25);

        Int32 Months = EndDate.Month - StartDate.Month;

        Int32 Years = EndDate.Year - StartDate.Year;

        Int32 Days = EndDate.Day - StartDate.Day;

        if (Days < 0)
        {
            Months = Months - 1;
        }

        if (Months < 0)
        {
            Years = Years - 1;

            Months = Months + 12;
        }
        
        string Ages = Years.ToString() + " Year(s) " + Months.ToString() + " Month(s) ";

SQL版本：

declare @dd smalldatetime = '1980-04-01'
declare @age int = YEAR(GETDATE())-YEAR(@dd)
if (@dd> DATEADD(YYYY, -@age, GETDATE())) set @age = @age -1

print @age  

==常见说法（从几个月到几岁）===

如果您只是为了通用，以下是代码作为您的信息：

DateTime today = DateTime.Today;
DateTime bday = DateTime.Parse("2016-2-14");
int age = today.Year - bday.Year;
var unit = "";

if (bday > today.AddYears(-age))
{
    age--;
}
if (age == 0)   // Under one year old
{
    age = today.Month - bday.Month;

    age = age <= 0 ? (12 + age) : age;  // The next year before birthday

    age = today.Day - bday.Day >= 0 ? age : --age;  // Before the birthday.day

    unit = "month";
}
else {
    unit = "year";
}

if (age > 1)
{
    unit = unit + "s";
}

测试结果如下：

The birthday: 2016-2-14

2016-2-15 =>  age=0, unit=month;
2016-5-13 =>  age=2, unit=months;
2016-5-14 =>  age=3, unit=months; 
2016-6-13 =>  age=3, unit=months; 
2016-6-15 =>  age=4, unit=months; 
2017-1-13 =>  age=10, unit=months; 
2017-1-14 =>  age=11, unit=months; 
2017-2-13 =>  age=11, unit=months; 
2017-2-14 =>  age=1, unit=year; 
2017-2-15 =>  age=1, unit=year; 
2017-3-13 =>  age=1, unit=year;
2018-1-13 =>  age=1, unit=year; 
2018-1-14 =>  age=1, unit=year; 
2018-2-13 =>  age=1, unit=year; 
2018-2-14 =>  age=2, unit=years;