I was 8 years old. I had to do it on the sly because my parents wanted me asleep by 8pm. And Star Trek was on at 8PM back in the 70's. I often got caught, because I couldn't contain my enthusiasm for the show. The episode I distinctly remember watching was "Balance of Terror" because I remember Kirk looking at a comet on the viewscreen. I've had my revenge many years later when I bought all three TOS seasons on DVD!