How old and how large has really nothing to do with being a startup or not . It just means a fledgling business whose revenues are expected to change rapidly and is developing a product/ business .

This is in contrast to stable or blue chip like companies who have stable revenues and their product lines are very well established.

The distinction is important for two groups of people .

a) investors - Risks and rewards are fundamentally different , therefore valuation methods are also different t you would use something like a Discounted Cash flow for stable corp you might look at IP, ARR , DAU or any number of other metrics for a startup whose revenues are not stable enough to use DCF etc

b) employees - change or innovation is far more deeply grained culture in startups , What worked yesterday may not work today, that requires different mindset to be successful at

Amazon , Uber are still startups despite what you think about their sizes , ages and current revenues . Building a AWS is only possible if you can completely rethink what is that you sell .