When is a startup no longer a startup?

Coming from the perspective of someone who has worked for venture capital for a decade, I do agree that the word 'start-up' seems to be thrown around carelessly. There are small companies who have been around for 8 years, are a two-person show, and they call themselves a start-up. On the other side of the spectrum, there are companies who make well over US$100m in topline and over 500 employees, but they would also like to say that they 'operate like a start-up'.
Techcrunch writer Alex Wilhelm took a stab at this back in 2014 with his piece on 'WTH is a startup anyway', where he defines that any company that achieves the following is NOT a start-up:

• $50 million revenue run rate (forward 12 months);
• 100 or more employees;
• Worth more than $500 million, on paper or otherwise.

Source: https://techcrunch.com/2014/12/30/what-the-hell...
But that said, if you are asking as a potential employee, then the culture and fit matters much more than how many employees the company has, and how much revenue run rate it's making.
Things you should probably consider include:

1. What's the dynamics of the team that you will be directly working with? Some people can appear to be nice/friendly, but turn out to be the most political and chauvinistic person (spoken from real-life experience). In a start-up, you want to be with people you can fight in the trenches with (read: spend late nights working on difficult problems with).
2. If it's still a small team, get a sense of what the founder's goals are, and his roadmap to take the team there.
3. Also, look at what the start-up is spending on. If it's splashing money on fancy offices, and expensive roadshows that don't provide ROI, you can be sure that there will be pressure if funding runs out.
4. What will you be responsible for, and how much opportunity is there to grow in this role? Usually, in start-ups, there is a lot of room for growth. But always good to get a sense of what this is.

I think it honestly depends on the founders' own attitude and definition.

I remember meeting up with Qoo10 team and this senior guy was proclaiming Qoo10 is still a start-up. That was when Lazada and Shopee just started and Qoo10 was the big brother then. So it sounds strange that Qoo10 senior team is labelling itself as a start-up and still trying new, innovative way to do things.

I tried thinking of these

1) length of time

2) profitability

3) market size

4) bootstrap or plenty of operating capital

Unfortunately, I can't think of one that can have a good definition of what's a start-up.

I think as long as the operating team are still trying to find ways to make things work and make them better, they can still be start-up?

My own company is only a few years old, but luckily I bootstrapped and insist on generating profit from Day 1. Now we are comfortable with operating capital, but we are still so shy from expanding relentless (actually we did try once and the scaling operating costs frightened me) so I do hope I can adopt this start-up all the way till it is a big size and people stared at me as I stared at the Qoo10 guy when he said its still a start-up