I know I was in kindergarten. I knew what Star Trek was. I knew Kirk, Spock, and Scotty (when I saw a re-run of Mirror Mirror I thought Scotty was trying to kill Kirk, because all red-shirts were Scotty). I have a vague recollection that there was a Star Trek cartoon that was on when I was watching Land of the Lost, Valley of the Dinosaurs, and Ernie Devlin.
Then I became more aware of Star Trek through the live action show and the Colorforms set (seriously). My next experience of TAS was the Viewmaster set of Yesteryear (around 1976). So I had that pretty well memorized.
I MAY have seen some in 1978ish.
But I don't think I really got to see TAS again until Nickelodeon ran them again in 1986. By then I could tell that Doohan and Barrett did all the voices.
But hey, it was before TVH and a year before TNG. I took any Star Trek I could get. (I'm a TAS fan, btw.)