20s being hyper significant is a psyop (I am half-joking). It can matter what you do early on, but main thing is make sure to look after your health to the extent that you can (like flossing your teeth and such) because habits that neglect your health will do damage over time even if it doesn't seem like it matters as much when you're younger. And the more embedded unhealthy coping mechanisms become, the harder it will be to change them when you're busy and dealing with stressful adult shit.

The rest of it is more variable than people make it out to be. I know somebody who started law school in his twenties but couldn't finish it then cause of the cost and finished it later in his forties (after lots of career stumbling about from here to there to make ends meet).

There are people who get married in their twenties and seem like they have their life together, but then get divorced later after kids in the picture make everything more complicated.

If you live under an individualist, capitalist state, expect some turbulence and understand that no matter what choices you make, there isn't really such a thing as an ideal life (at least not under that hellscape of a system). You can "do everything right" and still have things go wrong. You can do lots wrong and still have things work out in the end. Just do something, so that when you're looking back, you can say, hey, I did that stuff during my twenties. It doesn't need to be super significant or conventionally standout. Just make sure you're living rather than hiding away from life; I say that as someone diagnosed with generalized anxiety disorder. But even I still have stuff I did in my 20s that I can look back on as something, even if it wasn't always meeting conventional milestones.

If I could do my 20s over again (and make intention reality - sometimes willpower part is hard), I'd probably get more exercise, eat less crappy food, have a regimented sleep schedule, and seize the means of production.