I was about 5, it was during the final season of TOS.
I don't know what show my folks were watching but I distinctly remember walking into the living room from my bedroom when I heard the theme song start and getting a rush when I heard Shatner's "Space..the Final Frontier..."
Then my dad got up and quickly flipped the channel and said "I'm not watching that space s*^t!!!"
It wasn't until TAS came on Saturday mornings that I got re-introduced and then I was hooked. I searched out the late night re-runs and the rest is history!