Dynamically add n number of Rows in ASP.Net GridView with TextBox and DropDownList using C#

Dynamically added dropdownlist in GridView based on TextBox value using C# not working well

What i did was to add dropdown list and the code stop working because i think am not inputing the right code.

Originally when you click the add button it is suppose to add (n numbers) of 2 text boxes. But i added a dropdown list so that when you click on add button it adds (n numbers) 2 textboxes and dropdownlist.

My code

public partial class Ast : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (this.GridView1.Rows.Count > 0 && Session["myDatatable"] != null)
        {
            DataTable dt = (DataTable)Session["myDatatable"];
            dt.Clear();
            foreach (GridViewRow row in this.GridView1.Rows)
            {
                string firstName = (row.FindControl("txtFirstName") as TextBox).Text;
                string lastName = (row.FindControl("txtLastName") as TextBox).Text;
                string age = (row.FindControl("ageperson") as DropDownList).Text;
                dt.Rows.Add(firstName, lastName,age);
            }
            GridView1.Visible = true;
            GridView1.DataSource = dt;
            GridView1.DataBind();
            Session["myDatatable"] = dt;
        }
    }

    public void AddNewData()
    {
        DataTable dt = new DataTable();
        if (Session["myDatatable"] != null)
        {
            dt = (DataTable)Session["myDatatable"];
        }
        else
        {
            dt.Columns.Add("FirstName");
            dt.Columns.Add("LastName");
            dt.Columns.Add("age");
        }

        DataRow drow = dt.NewRow();
        drow["FirstName"] = "";
        drow["LastName"] = "";
        drow["age"] = "";
        dt.Rows.Add(drow);
        Session["myDatatable"] = dt;
    }

    public void BindGridview()
    {
        if (Session["myDatatable"] != null)
        {
            DataTable dt = (DataTable)Session["myDatatable"];

            if ((dt != null) && (dt.Rows.Count > 0))
            {
                GridView1.Visible = true;
                GridView1.DataSource = dt;
                GridView1.DataBind();
            }
            else
            {
                GridView1.Visible = false;
            }
        }
    }

    protected void Add(object sender, EventArgs e)
    {
        for (int i = 0; i < Convert.ToInt32(txtNo.Text); i++)
        {
            AddNewData();
        }
        BindGridview();
    }
    protected void OnSave(object sender, EventArgs e)
    {
        foreach (GridViewRow row in GridView1.Rows)
        {
            string firstName = (row.FindControl("txtFirstName") as TextBox).Text;
            string lastName = (row.FindControl("txtLastName") as TextBox).Text;
            string age = (row.FindControl("ageperson") as DropDownList).Text;
            string conString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
            string query = "INSERT INTO Student VALUES(@First,@Last,@age)";
            using (SqlConnection con = new SqlConnection(conString))
            {
                using (SqlCommand cmd = new SqlCommand(query))
                {
                    cmd.Connection = con;
                    cmd.Parameters.AddWithValue("@First", firstName);
                    cmd.Parameters.AddWithValue("@Last", lastName);
                    cmd.Parameters.AddWithValue("@age", age);

                    con.Open();
                    cmd.ExecuteNonQuery();
                    con.Close();
                }
            }
        }
        Session["myDatatable"] = null;
        Response.Redirect("material.aspx");
    }
}

<asp:TextBox ID="txtNo" runat="server" />
<asp:Button ID="Button1" Text="Add" OnClick="Add" runat="server" />
<hr />
<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false" Width="315px">
    <Columns>
        <asp:TemplateField HeaderText="First Name">
            <ItemTemplate>
                <asp:TextBox ID="txtFirstName" runat="server" Text='<%# Eval("FirstName") %>' />
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Last Name">
            <ItemTemplate>
                <asp:TextBox ID="txtLastName" runat="server" Text='<%# Eval("LastName") %>' />
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Age">
            <ItemTemplate>

                <asp:DropDownList ID="ageperson" runat="server" Height="25px" Text='<%# Eval("age") %>'
                    Style="margin-left: 0px" Width="416px"
                    AutoPostBack="True" Font-Names="Century Gothic" Font-Bold="True">
                    <asp:ListItem Selected="True" Value="1">Offline Die</asp:ListItem>

                    <asp:ListItem Value="3">Cylinder</asp:ListItem>
                    <asp:ListItem Value="4">Cutting Die</asp:ListItem>
                    <asp:ListItem Value="6">Embossing</asp:ListItem>

                </asp:DropDownList>

            </ItemTemplate>
        </asp:TemplateField>

    </Columns>
</asp:GridView>
<asp:Button Text="Save" runat="server" OnClick="OnSave" />

please help