Whether I'm nutty (which I know and accept) or it's confirmation bias, I tend to notice several people that I know who are about to turn 30 or are 30 who come to an especially challenging fork in the road.
An obstacle or series of events in their life, more than at any other point, that brings on self-doubt or insecurities or a general existential funk that seems to creep in out of nowhere. A soul search begins if they set time aside to understand and be patient with how dragged out it feels. Like a ball-and-chain at the bottom of the ocean, some sink into despair and go to extremes, and some don't make it out of the darkness. The '27 Club' comes to mind.
There's no formula or exact moment when it starts or finishes, but as I experienced, it's felt by only the individual and it's a crisis of confidence, but a blessing in disguise because it forces you to double down on what you value in yourself and others and to commit to something that maybe you've held off on or had doubts about. The decisions made at this time influence the years ahead, not to be cliche, but too late.
There's no instant gratification here; it may feel like a mile is taken for every inch, but keeping your chin up despite it all only sets you up later to feel empowered once it subsides; you're forced to take the long way home so that the lesson gets drilled in (and that the tough love was worth the struggle). A rites of passage into the adult realm, but still encouraged to be young at heart.