With the massive success of the "Iron Man" films, it was expected of Robert Downey Jr. to keep extending his contract and continue playing the titular role. However, according to an interview with Empire, it appears he turned down a fourth "Iron Man" film to make "Captain America: Civil War." 

"They said to me, 'If we have you, we can do this, or Cap 3 has to be something else.' It's nice to feel needed. And at this point it's about helping each other, too," the actor stated during the interview, as quoted by Collider. "I look at it as a competition and I go, 'Wow, maybe if these two franchises teamed up and I can take even a lesser position, with people I like and directors I respect, maybe we can keep things bumping along." 

If it wasn't for RDJ opting to go with the Civil War storyline then "Captain America 3" would have had to be reworked from the ground up. This decision also amplifies the rumor that he might be replaced by a younger character if "Iron Man 4" is green lit. 

Air Herald recently posted a theory that Ty Simpkins, the boy who played Harley Keener in the last "Iron Man" film, would be old enough by 2020 to take over the reins. Perhaps RDJ may even have a short cameo appearance as he hands over the Iron Man suit and responsibilities. 

Robert Downey Jr. would be turning fifty-four by the time "Avengers: Infinity War Part 2" rolls out in theaters in 2019. Just as Liam Neeson recently stated he would be quitting action films due to his age, RDJ might also be looking into less physically-demanding roles after Marvel's Phase 3.