It's no secret that Iron Man is the most successul solo Marvel Universe hero. Iron Man 3 made more than 1.2 billion dollars at the global box office- Robert Downey Jr and his superpowered suit of iron is movie moneymaking gold.

So naturally, you'd think that Marvel would have plans to continue Iron Man past those three films with "Iron Man" in the title. And yes, he's already got Avengers: Age of Ultron and Captain America: Civil War in his future, but what about another solo Iron Man film? It's possible- but not the way you think.

Robert Downey, Jr. is turning 50 this year, and the Marvel Cinematic Unvierse has already been scheduled all the way through 2019, when Downey Jr. will be 54 years old. That means that the earliest we could see Iron Man 4 (presuming Marvel schedules it at the very beginning of Phase 4) is in 2020, with a 55-year old Robert Downey Jr. No offense to the man, but that may be the point where it's no longer feasible to headline a multimillion dollar superhero action movie (although Harrison Ford's certainly done plenty of action flicks in his old age).

That means Marvel only has two options for a potential Iron Man 4. They write a storyline involving an older Tony Stark... or they just recast the character entirely. It's bound to happen sometime- these characters are insanely popular, and if Marvel wants the MCU to stretch on for decades, they'll have to admit that eventually, roles will probably need to be recast.

And the most obvious choice would be after Avengers: Infinity War. At that point, they'd be startin Phase 4 with a brand new slate, and there's no better way to show off a slate than with a new cast. Also, it's possible that Infinity War could involve some kind of time or reality manipulation from Thanos and his Infinity Gauntlet, which could be an easy way to write in a new Tony Stark. People love Iron Man- something's got to shake things up at some point.

Avengers: Infinity War Part 1 will premiere on May 4, 2018 and Avengers: Infinity War Part 2 will premiere on May 3, 2019.