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。

==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; 

我有一个定制的计算年龄的方法，加上一条奖金验证消息，以防有帮助：

public void GetAge(DateTime dob, DateTime now, out int years, out int months, out int days)
{
    years = 0;
    months = 0;
    days = 0;

    DateTime tmpdob = new DateTime(dob.Year, dob.Month, 1);
    DateTime tmpnow = new DateTime(now.Year, now.Month, 1);

    while (tmpdob.AddYears(years).AddMonths(months) < tmpnow)
    {
        months++;
        if (months > 12)
        {
            years++;
            months = months - 12;
        }
    }

    if (now.Day >= dob.Day)
        days = days + now.Day - dob.Day;
    else
    {
        months--;
        if (months < 0)
        {
            years--;
            months = months + 12;
        }
        days += DateTime.DaysInMonth(now.AddMonths(-1).Year, now.AddMonths(-1).Month) + now.Day - dob.Day;
    }

    if (DateTime.IsLeapYear(dob.Year) && dob.Month == 2 && dob.Day == 29 && now >= new DateTime(now.Year, 3, 1))
        days++;

}   

private string ValidateDate(DateTime dob) //This method will validate the date
{
    int Years = 0; int Months = 0; int Days = 0;

    GetAge(dob, DateTime.Now, out Years, out Months, out Days);

    if (Years < 18)
        message =  Years + " is too young. Please try again on your 18th birthday.";
    else if (Years >= 65)
        message = Years + " is too old. Date of Birth must not be 65 or older.";
    else
        return null; //Denotes validation passed
}

方法调用此处并传递日期时间值（如果服务器设置为美国语言环境，则为MM/dd/yyyy）。将其替换为消息框或要显示的任何容器：

DateTime dob = DateTime.Parse("03/10/1982");  

string message = ValidateDate(dob);

lbldatemessage.Visible = !StringIsNullOrWhitespace(message);
lbldatemessage.Text = message ?? ""; //Ternary if message is null then default to empty string

记住，您可以按任何方式格式化邮件。

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;
}

我认为TimeSpan包含了我们所需要的一切，而不必求助于365.25（或任何其他近似值）。扩展Aug的示例：

DateTime myBD = new DateTime(1980, 10, 10);
TimeSpan difference = DateTime.Now.Subtract(myBD);

textBox1.Text = difference.Years + " years " + difference.Months + " Months " + difference.Days + " days";

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;
    }
}