Plus Fog Creek is more on an anti-start-up since it explicitly did not want to grow fast at all costs. That's why Trello was spun off.

> How long does a company have to be around until it's no longer considered a start-up?

My simple personal guide is: when it has been around for more than 2 years, has found a viable business strategy or is certain to be around for 4 more years.

There is a discussion on this question here: https://www.quora.com/When-does-a-startup-stop-being-a-start...

I think a good definition of what a startup is would consider rate of growth (now and projections into the near future) rather than length of time since inception.