Marvel Studios has made some bold and brave decisions in the years since the first Iron Man movie in 2008. They've gone with unproven directors like Kenneth Branagh and Shane Black and they've banked on stars with limited profile and made them into superstars.But one of their biggest moments was when they brought on Joss Whedon back in 2010. The writer and director was previously best known for creating and running Buffy and had only directed a single feature - 2005's Serenity. Yet he not only was trusted to craft 2012's Avengers but also become a touchstone for the entire Marvel cinematic universe, working on rewrites of the various other movies in production and maintining a consistent tone throughout the interconnected features.That's a huge job, and one that was always going to last for a limited time. Now we have confirmation that Whedon will be walking away after Age of Ultron, with The Winter Soldier directors Joe and Anthony Russo instead taking the helm for the two-parter Avengers Infinity War. That's sad news in some ways but Whedon basically admits that he's exhausted from working on two of the biggest films of all time for 5 years: "Every movie I have ever made has been an ensemble piece of increasingly enormous proportions. That many balls in the air, it’s only going to get bigger with Infinity War. I’m not going to be able to give it what I would need to. It’s a young man’s game.”While we'll miss the attention to detail Whedon brings to this kind of process, it's not at all surprising that he's dropping off the Marvel map. People seem to forget that he was taken on for a 5 year contract with the comic-book makers and that's set to finish in 2015. It must have been a huge decision for him to give that kind of time to a franchise, to come on when it was in the early days and help shape it into one of the biggest entertainment brands in the world.