there is direct evidence that disorders like schizophrenia and bipolar tend to manifest in the early 20s. generalizing that to everyone's experience of their 20s is obviously a bit tenuous

The last of your prefrontal cortex's major systems come online in your earlier 20s, which can lead to bipolar or schizophrenia if those systems are disordered. In most, it just happens to lead to "your first deep thoughts about the broader world and your significance in it" and may lead to writing youthful cringepieces like "The Conscience of a Hacker". It's kind of like a second puberty. The underlying cause is the same, if the results are vastly different.