Show or hide form element
===

I have one html form - On that one text field like

    <input type = "text" name = "age" value = "" id = "txt_age">

On the value of age i have to show this element like -

    <select name = "married">
      <option value = "Yes">Yes</option>
      <option value ="No">No</option>
    </select>

If age>18 then married field will be shown to form otherwise hide it.This age>18 condition is stored in database.As value of age changes married field toggle(show/hide).
How can i do it with javascript/jquery.Please suggest.