I love Robert Downey Jr. as Tony Stark. If only because he is extremely entertaining. Physically he is just fine (age and appearance). But he won't be doing this forever. If, god forbid, he refuses to hop on board with Avengers 2 or Iron Man 4, who should take his place?
Now I'm sure there have been "Cast Stark" threads in the past, but this is less "Cast Stark" and more "Fill RDJ's shoes."
I honestly have no idea or I would take a shot at it.