I, too, caught a glimpse or two of the show in the late '60s, but I didn't start watching regularly until 1970.
I guess you could say the first wave of fans were those who saw the show from the beginning. The second wave could be those fans who started watching when it went into syndicated reruns throughout the '70s. And I would consider myself one of the second wave. The third wave could be those who got into the show after seeing one or more of the films, and admittedly the line would get blurry here. I've known of some folks who saw the films and had no idea they were spun off a decades old television series.
Any subsequent wave of TOS fans could have come from being introduced to Trek through any one of the spin-off series until finally you get to JJtrek.