My first ep was The Doomsday Machine when I was 4 years old. I was terrified, absolutely frozen in fear. I still get a whiff of that feeling when I hear the TOS scary music.
I am proud to say that my kids have seen ALL of Trek. Maybe a little patchy on TAS but otherwise all of it. I get pretty irritated when I read on these forums people saying they can't hack TOS because it's "too old" when I've got a teenage boy who reveres it.
Start them young, suck them in.