New York Times - Economix: Dimming Optimism for Today’s Youth
Unfortunately, this data set only goes back to 1983. I'm quite certain that if you asked this question in the 1979-1981 era you would see similar low numbers. In fact, I'm pretty sure I remember such survey data being reported by other groups at that time.
I think the response to this question is more reflective of a mood than an indicator of what the future will be. I have virtually now doubt that youth will live better than their parents over the course of their respective lives. I'm doubtful they will live happier lives. (See yesterday's post.)