Frankly age isn't an issue. Are we saying Ford is too old or the fictious character of Indiana Jones is too old? Ford has never denied that he uses stunt doubles on the most dangerous stunts. So his health isn't at risk. Critics who say we aren't sure we want to see an "old" Indiana Jones there's this. The fans WANTED a 4th Indiana Jones, so now they are complaining he's too old? The character has drinken from the Holy Grail, so they could set this movie in the year 3000, Indiana Jones will still be alive. Old, but alive. The movie is done filming and Ford didn't get injured, so the stunts he did do for "Kingdom of the Crystal Skulls" went fine, or we would have heard otherwise. Look no matter how you cut this Ford is not going to look like he did on "Raiders", it just isn't going to happen. However seeing the pictures that were released there was one pic where he actually didn't look any older then he did in "Last Crusade".
http://www.theraider.net/films/indy4/gallery/makingof/mo_04.jpg