isn't a midlife crisis is just a label for burnout / depression at the end of the day? everybody who is depressed feels like life is stuck and going nowhere.
I would distinguish a mid-life crisis (or in the positive case, mid-life inspiration) by the realisation that you don't have infinite time left to do everything you dream to do.
In your 20s, it's easy to think that your entire life is still ahead of you, and you will have time for everything. It doesn't matter if you spend a few years in a mediocre relationship, a career that won't work out in the long run, on a terrible startup idea, and so on.
Somewhere in your 30s you realise that you do need to get started on what you really dream to do, or you might never have the chance to. Whether it's a crisis or inspiration depends on how far off your previous life was from it.