Hi ,
You can use RangeValidator
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:RangeValidator ID="RangeValidator1" runat="server"
ControlToValidate="TextBox1" Display="Dynamic" ErrorMessage="less than 18 "
MaximumValue="1/1/2080" MinimumValue="01/01/1994" SetFocusOnError="True"
Type="Date"></asp:RangeValidator>
Best Regards
M.Mitwalli