Startup means high scale business with the aim to build new technology and/or test new business model at the beginning. Beginning where you have nothing, you only loose and stress a lot, you are not sure what do you do, what works and what doesn't.
As soon as you returned investors all the money (ROI) in at least x3 amount and you have profitable business, e.g.
you are no more a startup.
At the end it depends on your own definition of a startup. There is no clear standard of what startup is. If startup for you means keep moving forward, changing yourself and innovating, then you may be a startup forever.