how to display the current age , months , and days by entering the date of birth?

Read this article by OriginalGriff:
Working with Age: it's not the same as a TimeSpan!]

I won't give you the algorithm because that something you should be able to think about by your self

use DateTime:

DateTime today
DateTime enteredBirth

today = DateTime.Now.Date, then you have the current date.
then simply "..." enteredBirth date from today.

voila you recieve a DateTime with Years,Months,Days,hours,minutes,seconds
and this one is your Age in years, months and days

Finally i found :
xxx.aspx

<table class="style1">
                        <tr>
                            <td align="center">
                                ENTER YOUR DATE OF BIRTH</td>
                        </tr>
                        <tr>
                            <td align="center">
                                <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
                            </td>
                        </tr>
                        <tr>
                            <td align="center">
                                &nbsp;</td>
                        </tr>
                        <tr>
                            <td align="center">
                                years :
                                <asp:Label ID="lblyr" runat="server" Text="Label"></asp:Label>
&nbsp;&nbsp;&nbsp;&nbsp; months :&nbsp;
                                <asp:Label ID="lblmnths" runat="server" Text="Label"></asp:Label>
&nbsp;&nbsp;&nbsp; days :&nbsp;
                                <asp:Label ID="lbldays" runat="server" Text="Label"></asp:Label>
                            </td>
                        </tr>
                        <tr>
                            <td align="center">
                               &nbsp;</td>
                        </tr>
                        <tr>
                            <td align="center">
                                <asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="submit" />
                            </td>
                        </tr>
                    </table>

xxx.cs

protected void Button1_Click(object sender, EventArgs e)
{

DateTime dob = Convert.ToDateTime(TextBox1.Text);
int birthmonth = Convert.ToInt32(dob.Month);

DateTime today = DateTime.Today;
int month = Convert.ToInt32(today.Month);

int age = today.Year - dob.Year;
int currentmonth = month - birthmonth;
int currentmonth2 = birthmonth - month;


if (birthmonth > currentmonth)
{
lblyr.Text = age.ToString();
lblmnths.Text = currentmonth2.ToString();

}
else
{
lblyr.Text = age.ToString();
lblmnths.Text = currentmonth.ToString();
}
}