After high school or college the training wheels are completely off and the guard rails imposed by parents and schools are essentially gone. You’re no longer surrounded by people of similar socioeconomic backgrounds and especially age.
Nobody’s telling you that the “successful relationship” assignment is due by 30, “marriage” by 32, “kid(s)” by 35, etc.
You’re on your own. You don’t have to conform socially anymore, you can get away with ignoring rules. Maybe it works and you become a trendsetter, or maybe you end up in a tent under a bridge. A lot of what happens is luck, good or bad, and a lot of that luck is going to come from how well off your family was when you were young. You’re far more likely to be exposed to other people’s randomness, too, and that can push you up the ladder quickly or wreck you.
There’s variables that you can control and even more that are outside your control once the guard rails are gone that will make your life very different from your peers.