I suspect that this particular environment selected for a significantly higher-than-average degree of emotional maturity for that age range.

The lore — I have no sources for this; it was word-of-mouth at the time — was that when psychologists had once tested the student body for emotional maturity, what they found was that entering students were no more mature than comparable teenagers, but that graduating students were as mature as other college graduates. IOW, it was believed to be not a selection process, but an "if you treat 'em like adults, they'll act like adults" process.

(Of course, this also neatly fits the institution's founding ideology, which was opposition to the sustained infantilization of mainstream schooling.)