Allow only numbers in textbox

function isNNumeric(e)
{
var isNN = (navigator.appName.indexOf("Netscape")!=-1);
var keyCode = (isNN) ? e.which : e.keyCode;
//alert(keyCode);
if (isNN)
{
if (keyCode == 0)
return true;
}
if((keyCode>47&&keyCode<58)||(keyCode==8)||(keyCode==9))
{
return true;
}
else
{
if (e.returnValue)
{
e.returnValue = false;
return false;
}
else if (e.preventDefault)
{
e.preventDefault();
return false;
}
this.event.returnValue = false;
return false;          
}
}

  <asp:TextBox ID=”txt_maxage” Onkeypress=”return isNumberKey(event)” runat=”server”></asp:TextBox>