Robert Downey Jr. was nominated to Oscar in 1992, at the age of 27, he didn't win though. There was a joke about him in Simpsons in 1999, so he was already a well known name far before Iron Man. https://www.youtube.com/watch?v=UhjHVQsJOlo

His life is more like an example of how people can get back to the top after years of heavy drug abuse.

Martha Stewart wrote her first book at the age of 42, I would count that as the start of her career.

This chart is a bit confusing, sometimes it lists when people started a later flourishing business, sometime when they were at the top.

Don't expect to do anything cool if you're 46 tho

Yeah, that's not helpful. Almost all these people were working towards their big thing many, many years earlier.