I don't know the rules of royal succession, but since the Queen isn't in any hurry to retire, I'm going to guess that Charles might at some point become too old for the job; if that occurs, I think the older son, William, would be king. I wouldn't mind that -- from what I heard Williams is a sensible young man. And he looks a heckuva lot better than the rest of the royals.
The Brits ought to just make Madonna Queen and get it over with already.
The Queen is eighty, her son is fifty-seven. Even if she lives to be 100, he would still be "young enough" to become king at seventy-seven. An age restriction has never been put into force in the past.