I think the criterion I like best is that if you're actively sacrificing profitability for growth, you're a startup.

Obviously there are exceptions in both directions, and other factors to consider, but this seems to be a pretty good starting point for discussion to me.

Unfortunately, declaring your company a start up is sexy. My last company was about 130 employees, made some profit but mostly enough to keep the business afloat, been around for a little over 10 years, and was reluctant to call itself a start up.

The company I'm currently at has close to 1,200 employees, been around for close to 10 years, its own free cafeteria, profitable enough to fund several high-risk initiatives, but easily calls itself a start up..

My personal opinion: you're no longer a start up if you're able to afford your own "free food cafeteria". ;-)

When you have a repeatable business model. Uber has repeat customers but is a long way from having figured out how to make the profits required by their business and funding history.

When you IPO.