I'm 38 so I'm technically under 40. I have always known about the 1975 issue because, to save face, many JW parents of people my age make it into a big joke on themselves. My parents do this.
My folks: Tee Hee! We planned when to have you so that you'd never have to go to school. Most of us felt that 1975 would be the end. Silly us! Well, it's our fault from running ahead of the organization.
I know of about a dozen other people I grew up with whose parents had done the same thing; planning their birth so that they would be pre-school aged children when 1975 came.
I also know of a few who went heavily into debt around this time (like late '73 to '74). They figured they would never have to repay the money. There were a lot of people driving new cars, indulging in luxuries, etc. Although the "materialism" was frowned upon, it was winked at because everyone believed Armie was on its way.
When it didn't come, there were a lot of janitors and window washers saddled with excessive debt that they couldn't repay.
I remember several people in my area left the bOrg over this.
Of course, true to WT doctrine, they were viciously labeled apostates and told it was their fault for running ahead of the organization and "overly anticipating" Jehovah.
Plausable deniability has always been established in the bOrg because while JWs are expected to obey the FDS, the JWs are also told that the GB are imperfect men and thus subject to imperfection and error.
Classic case of Orwellian Doublethink.
Those who didn't turn "apostate" and didn't leave the bOrg were however very bitter over the whole thing. I very much remember stern lectures from COs and DOs scolding the congregation for murmuring, allowing their zeal to cool, and "grieving the spirit."
On a depressing end-note, the 1975 disaster had been stomped out of the R&F active memory by 1980 and hardly anyone spoke about it anymore. If they did, they were very quickly reprimanded. To even mention 1975 usually led to a JC.
During my teenage years, my parents very reticent to even explain the whole thing to me.