Through Javascript Textbox to show error Age less than 18

function checkDate(sender,args) { //create a new date var and set it to the //value of the senders selected date var selectedDate = new Date(); selectedDate = sender._selectedDate; //create a date var and set it's value to today var todayDate = new Date(); var mssge = "";   if(  todayDate -selectedDate >=18) { / alert("Warning! - Date of Birth  is  valid  "); } }     Date Of  Birth             "width: 292px">                          "TextBox3" runat="server"  ValidationGroup="a">  "CalendarExtender2" targetcontrolid="TextBox3"  runat="server" OnClientDateSelectionChanged="checkDate">>                    "ValidatorCalloutExtender3" runat="server" TargetControlID="RequiredFieldValidator3">                                                  "RequiredFieldValidator3" runat="server"                      ErrorMessage="RequiredFieldValidator"  Display="None"                      ControlToValidate="TextBox3" SetFocusOnError="True" ValidationGroup="a">               "Button1" runat="server" Text="Insert"  ValidationGroup="a" onclick="Button1_Click"                      style="width: 48px"  />                    "Label2" runat="server" ForeColor="#FF6600"                      Text="Data  Sucessfully Inserted "> Now   is   coding  inside  button    protected void Button1_Click(object sender, EventArgs e)     {         try         {             string constr;             constr = WebConfigurationManager.ConnectionStrings["JAPITConnectionString"].ConnectionString;             SqlConnection con = new SqlConnection(constr);               con.Open();             string str = "insert   into  Doe_detail values('" + TextBox1.Text + "','" + TextBox2.Text + "','" + TextBox3.Text + "','" + TextBox4.Text + "','" + TextBox5.Text + "')";             SqlCommand cmd = new SqlCommand(str, con);             cmd.ExecuteNonQuery();             con.Close();             Label2.Visible = true;             TextBox1.Text = "";             TextBox2.Text = "";             TextBox3.Text = "";             TextBox4.Text = "";             TextBox5.Text = "";                   }         catch (Exception es)         {         }     }td>