At most, I could see them using a younger actor to set up the storyline in the opening sequence...I've heard Marion is back for Indy IV so maybe the opening could be a flashback to him meeting her and Abner, I don't know.
Lucas, Spielberg and Ford are all on the record that the film won't overtly deal with an aging adventurer, but it will figure in, in some minor capacity.
Besides, I fail to see what the big deal is with people's obsession with aging movie stars. John Wayne was 60 when he made True Grit. Lee Marvin was 62 in Delta Force. Harrison Ford was 40 in Raiders. What's the big deal?