Need to calculate the age from the select value

private void Page_Load(object sender, EventArgs e)
       {
           if (!IsPostBack)
           {
               ddlMonth.DataSource = Enumerable.Range(1, 12).Select(a => new
               {
                   MonthName = DateTimeFormatInfo.CurrentInfo.GetMonthName(a),
                   MonthNumber = a
                   //DataTextField = DateTimeFormatInfo.CurrentInfo.GetMonthName(a),
                   //DataValueField = a
               }
               );

               //If this code is not working then try the above code in the loop.
               ddlMonth.DataTextField = "MonthName";
               ddlMonth.DataValueField = "MonthNumber";

               ddlMonth.DataBind();
               ddlYear.DataSource = Enumerable.Range(DateTime.Now.Year - 99, 100).Reverse();
               ddlYear.DataBind();
               ddlday.DataSource = Enumerable.Range(1, DateTime.DaysInMonth(DateTime.Now.Year, Convert.ToInt32(ddlMonth.SelectedValue)));
               ddlday.DataBind();
           }
       }
       protected void ddlMonth_SelectedIndexChanged(object sender, EventArgs e)
       {
           ddlday.DataSource = Enumerable.Range(1, DateTime.DaysInMonth(DateTime.Now.Year, Convert.ToInt32(ddlMonth.SelectedIndex + 1))); ddlday.DataBind();
       }
   }