I also was a teenager in 1975. As someone else mentioned, it was the build up to 1975 in their books and other literature, starting with the freedom in the sons of God book 1966 (my mum still has that book).
It was all anyone talked about. I left school at 15 because of it. I remember going to friends' congregations and some of them had countdown charts on the wall!
The 1968 watchtower asked "Why are you looking forward to 1975". I wasn't really, I was scared, I didn't want anything to happen, but if it was going to as I was being led to believe then I wanted to be in what I thought was the 'only' place to be to survive. Looking back it all seems so ridiculous now that so many people could have believed this. We were deliberately led to believe this by their literature. It makes me mad when they all deny it was ever said.
That was the main reason I left. The constant denial that they never said it!