"Roughly half of all lifetime mental disorders in most studies start by the mid‐teens and three‐fourths by the mid‐20s." - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1925038/
There is still some element of truth to this, though, especially with bipolar disorder and psychosis. It puts me in mind of the fact that allergies can also manifest around 25... and makes me wonder if there's a connection there.
Getting over trauma and other emotional baggage has resulted in large improvements in sustained focus, lower inhibitions, and general cognitive improvements for me and I'm well past the age of 25.
Gradually worsened up until ~25, though. Vastly improved after a couple of years, and seeing the tail ends ~30
As a personal aside, it makes me question the sensibility of letting kids go off to war, vote, drink, or smoke before this age.
Why would that be "unsurprising"? It's not like mental illness is intuitively correlated with increases/improvements in brain functioning...
If anything, one would expect the inverse: for some mental illness the increase in brain functioning around mid-twenties would somewhat help reduce it.