And maybe it's just me, but I almost stop thinking of startups as 'startups' when you're at 40+ employees. I don't know if that's fair, or even rational, but it's much harder to be the same plucky young company with 40 employees that you were with three to five.
Probably I'm just being obnoxious though, because clearly there are cases where it's still 'obviously' a startup, but I'm having a hard time thinking of any that don't involve VC.
In my mind, startup-ness is not about size, but about culture. You've got invaders and you've got occupiers. Invaders have to improvise, have to be agile, have to seek out a better way because they have no incentive to uphold the status quo. Occupiers just keep doing what they already do, optimizing it perhaps, but never truly innovating. There are 40-person companies out there who've already become occupiers, and there is one 400 billion dollar giant who is arguably still in invasion mode.
I also like Eric Ries' definition: a group of people operating under conditions of extreme uncertainty.
Would you say by that definition, GitHub isn't a startup? (Since they're well established in their niche, despite being young, and have a huge subscriber base forking over wads of cash every month?)