I made the mistake of joining startups early in my career. I still suffer because of those decisions. It is much better to work for a big brand when you are young. Yes, in startups you get to wear a lot of hats but unless you are really lucky, you never learn anything in depth. Big companies have enough good people to learn from if you are motivated enough (as I was). And worst of all, unless you are one of the founders, and the startup is reasonably successful - then they will bring a "VP of Engineering" from a big company after two years irrespective of how much you have helped them build their systems and probably deserve that role.