Valid reasons to work for a startup: - You are a cofounder. - You have little experience and you are using this to break into the industry, and get experience on many different technologies ("wear many hats"). - They are working on a very specific problem or using a specific technology that you strongly desire to work on and it's difficult to do it anywhere else. - You want to work a certain way (remote, on the beach…
Not sure if going back to a startup would be nearly as beneficial in terms of "levelling up." Seems like the game now is to grind whiteboard before I get too old for Google.
A friend grew from junior developer to director of IT at that startup, so there's that aspect as well, though I have also seen a number of startups hire leadership outside.