Check out Don Bowen's 1977 article "Quantrill, James, Younger, et al.: Leadership in a Guerrilla Movement, Missouri, 1861-1865." He deals specifically with Quantrill's men and leaders of associated guerrilla bands (quite a few of which were splinters from his original group circa-1862-63). He pegs the average age of a guerrilla leader in Missouri at 23 and the average age of a rank-and-file bushwhacker at 20. Several were younger than 20 to bring the average so low (like Jesse James, Arch Clement, etc.)

MCH