Age: 
<asp:TextBox ID="TextBox1" Runat="server"></asp:TextBox>
  &nbsp;
<asp:CompareValidator ID="CompareValidator1" Runat="server" 
  Operator="GreaterThan" ValueToCompare="18" 
  ControlToValidate="TextBox1" 
  Text="You must be older than 18 to join" Type="Integer">
</asp:CompareValidator>
