Which, once again, is unknown. Not that I think Darkstar is Ned and Ashara's love child, but until we know his age THAT factor cannot be used to rule out even this remote possibility. Assumptions about Ser Gerold's age are not the same as knowledge of it. My own favorite wild guess concerning the Darkstar is that he is a Targaryen descended from Aerion - the mad prince whose child is passed over by the Great Council.