Please use
Range validator control to check the things. You can get the code to find the solution.
Client Side:
<asp:TextBox ID="txtDate" runat="server" />
<asp:RangeValidator ID="RangeValidator1" runat="server" ControlToValidate="txtDate" Type="Date" ErrorMessage="Please enter 15 years back date from the current date." />
Server Side:
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
{
RangeValidator1.MinimumValue = DateTime.Now.AddYears(-100).ToShortDateString();
RangeValidator1.MaximumValue = DateTime.Now.AddYears(-15).ToShortDateString();
}
}