Edit in JSFiddle

function validar(){
    var edad = Number(document.getElementById('edad').value),
        meses = Number(document.getElementById('meses').value);
    
    if(edad>6 && meses >= 11)
        alert('Datos válidos.');
}

<tr> 
<table>                
  <td width="32"><font size="3" face="Arial Narrow"><font color="#FF0000" size="2">Edad:</font></font> 
                  </td>
                  <td width="30"><font size="3" face="Arial Narrow"><font color="#FF0000" size="2"> 
                    <input name="edad" type="text" id="edad" value="0" size="5" maxlength="1">
                    </font></font></td>
                  <td width="14"></td>
                  <td width="69"><font size="3" face="Arial Narrow"><font color="#FF0000" size="2"> 
                    <input name="meses" type="text" id="meses" value="0" size="5" maxlength="2">
                     <td>
                         <input type="button" value="Validar" onclick="validar()">
                     </td>
                    </td>
               </tr>
</table>