Seth Rogen admits he hasn't grown up since he became famous.

The actor appeared in TV show Freaks and Geeks when he was a teenager, before getting his big break in 2007 comedy Knocked Up.

Since then Seth has appeared in a several hit movies, but admits he hasn't really matured.

"I probably just haven't changed much in the last ten years. I peaked early. I heard someone say once that you stop ageing at the age you got famous," he told the December issue of Esquire. "So I don't know, how old is that? I was maybe 24 or 25? That's not bad. I think I heard Ian McKellen say that and he was 85 years old at the time."

Seth's latest big screen outing is 50/50 a comedy which follows a 27-year-old guy who is diagnosed with a rare form of spinal cancer. The script was written by Seth's friend Will Reiser, who based the story on his own experiences.

Joseph Gordon-Levitt plays cancer patient Adam, while Seth is his best friend.

The movie has attracted controversy for its humour which is often close to the edge.

During one scene, Seth's character makes a joke about Patrick Swayze who died in 2009 after battling pancreatic cancer.

The 29-year-old comic star has defended the script, insisting test audiences responded well to it.

"We went there. We weren't sure that would fly," Seth admitted. "We test the movies with real audiences and there was more we expected people to react negatively to, but they didn't. So we left it in.".