Robert Downey Jr. is currently 47 years old, and shooting one of the most physically demanding roles that an actor can play, Iron Man. It's not a huge surprise that he would get injured, and actually a bit surprising that there haven't been reports of more injuries during filming of the first two 'Iron Man' movies. Reports say that the filming delay is only temporarily, as Downey sustained an ankle injury on the Shane Black directed sequel.
The official statement from Disney, which currently owns Marvel Studios is below:
Robert Downey Jr. sustained an ankle injury on the set of Iron Man 3 in Wilmington, North Carolina while performing a stunt. There will be a short delay in the production schedule while he recuperates.
They haven't said anything else, not announced a change in the release date, so it doesn't seem to be a major injury, and filming will likely only be delayed for a short while. Let's wish for Iron Man's speedy recovery, and hope nothing else goes wrong during the shoot.