Alpha Xi Delta's 10 founders ranged in age from 16 to 26. One, Eliza Drake Curtis Everton, was what today would be considered a "non-traditional" student. She was 26 and and a widow.

Kappa's six founders were between 14-22.

I agree, not many high schoolers would have the mindset that these women and men had at that age in creating their organizations. But also, many current 16 to 22-year-olds aren't being taught Classic Greek and Latin. I don't know what these schools are coming to nowadays! ;)

Founders
 

Zeta Psi Fraternity was started June 1st, 1847 in New York, NY at New York University by John Moon Skillman, William Henry Dayton, and John Bradt Yates Sommers. Their ages were 14,18,18.
Matt Lofgren
Zeta Psi Fraternity
Alpha Mu Chapter @ Dalhousie University

The founders of Phi Sigma Kappa ranged in age from 17-24, and the eldest, Henry Hague, was actually an Englishman.