Edit in JSFiddle

$("button").click(function(){
    var div = $(this).parent(),
        valor = $(div).find("input:radio:checked").val();
    
    if (valor)
        alert(valor);
});

<div id='miDiv'>
    <input type="radio" name="edad1" value="20" />
    <input type="radio" name="edad1" value="25" />
    <button>Clic</button>
</div>

<div id='miDiv2'>
    <input type="radio" name="edad2" value="18" />
    <input type="radio" name="edad2" value="15" />
    <button>Clic</button>
</div>