//aspx ファイル
<input id="Text1" type="text" runat="server"/><br/>
<input id="Button1" type="button" value="button" onclick="Button1_Click" />
//csファイル
protected void Button1_Click(object sender, EventArgs e)
{
//There are so many control.I am accessing value in string
string s1 = "Text1";
TextBox AgeTextBox = Page.FindControl(s1) as TextBox;
AgeTextBox.Text;
}