Pat Martino was a world-class professional at 15, pretty much dashing the "10-20 years to become a jazz player" meme to pieces. I was playing jazz in a major city for a living by the age of 24, having played the guitar for 9 years. The secret was as stated above: immersion in the music. Practicing technique on the classical guitar will develop enough technique to play jazz, but learning the aural language is key. I spent a couple of frustrating evenings with John Williams trying to get him, at his request, to play jazz, but he had too little jazz in his lifetime of listening, in spite of his father being a pretty good swing jazz player. Charlie Byrd, likewise, was a pretty good swing-jazz player, not particularly sophisticated or daring. As far as separating who's a real jazz guitarist and who isn't, that's an exercise of the listener's ego, period. There are, however, several excellent jazz guitarists utilizing the nylon-string guitar, including Gene Bertoncini, Romero Lubambo, Ralph Towner, and the late Bola Sete, Rafael Rabello and Baden Powell.
Lester Devoe Flamenco Negra
Lester Devoe Flamenco Blanca
Aparicio Flamenco Blanca with RMC pickup
Bartolex 7-string with RMC pickup
Giannini 7-string with Shadow pickup
Sal Pace 7-string archtop