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;

我经常用手指数。我需要看一下日历，以确定事情何时发生变化。这就是我在代码中要做的：

int AgeNow(DateTime birthday)
{
    return AgeAt(DateTime.Now, birthday);
}

int AgeAt(DateTime now, DateTime birthday)
{
    return AgeAt(now, birthday, CultureInfo.CurrentCulture.Calendar);
}

int AgeAt(DateTime now, DateTime birthday, Calendar calendar)
{
    // My age has increased on the morning of my
    // birthday even though I was born in the evening.
    now = now.Date;
    birthday = birthday.Date;

    var age = 0;
    if (now <= birthday) return age; // I am zero now if I am to be born tomorrow.

    while (calendar.AddYears(birthday, age + 1) <= now)
    {
        age++;
    }
    return age;
}

在LINQPad中运行此过程可获得以下结果：

PASSED: someone born on 28 February 1964 is age 4 on 28 February 1968
PASSED: someone born on 29 February 1964 is age 3 on 28 February 1968
PASSED: someone born on 31 December 2016 is age 0 on 01 January 2017

LINQPad中的代码在这里。

这里有一个非常简单且易于遵循的示例。

private int CalculateAge()
{
//get birthdate
   DateTime dtBirth = Convert.ToDateTime(BirthDatePicker.Value);
   int byear = dtBirth.Year;
   int bmonth = dtBirth.Month;
   int bday = dtBirth.Day;
   DateTime dtToday = DateTime.Now;
   int tYear = dtToday.Year;
   int tmonth = dtToday.Month;
   int tday = dtToday.Day;
   int age = tYear - byear;
   if (bmonth < tmonth)
       age--;
   else if (bmonth == tmonth && bday>tday)
   {
       age--;
   }
return age;
}

这可能会起作用：

public override bool IsValid(DateTime value)
{
    _dateOfBirth =  value;
    var yearsOld = (double) (DateTime.Now.Subtract(_dateOfBirth).TotalDays/365);
    if (yearsOld > 18)
        return true;
    return false;
}

我创建了一个Age结构，如下所示：

public struct Age : IEquatable<Age>, IComparable<Age>
{
    private readonly int _years;
    private readonly int _months;
    private readonly int _days;

    public int Years  { get { return _years; } }
    public int Months { get { return _months; } }
    public int Days { get { return _days; } }

    public Age( int years, int months, int days ) : this()
    {
        _years = years;
        _months = months;
        _days = days;
    }

    public static Age CalculateAge( DateTime dateOfBirth, DateTime date )
    {
        // Here is some logic that ressembles Mike's solution, although it
        // also takes into account months & days.
        // Ommitted for brevity.
        return new Age (years, months, days);
    }

    // Ommited Equality, Comparable, GetHashCode, functionality for brevity.
}

一句话的回答：

DateTime dateOfBirth = Convert.ToDateTime("01/16/1990");
var age = ((DateTime.Now - dateOfBirth).Days) / 365;