Most stations played it in early prime time, five nights per week. How is that not targeting that demographic?
I only have anecdotal evidence to offer, considering that I'm Generation-X and watched it on UHF in reruns in the 70s. My dad worked long enough hours that he was not home to watch it at the 5PM or so slot that it was in. My mom was busy cooking dinner. It was therefore not "appointment television" in this household. It was a show that nerdy boys watched after the afternoon cartoons were over and before sitting down at the dinner table. If the rest of the family were aware of Star Trek, it was more through the indirect influence of those in the household who made it part of their ritual to watch it.