I'm 53 and this was my experience too, although I didn't make the jump to Astra. I remember my dad using a DE before I was shaving but I think he'd switched to Trac II by the time I started (79-80ish). I don't remember seeing DE gear in stores but then I wasn't looking for it, and as others here have noted it was like toothbrushes, etc. The internet has really enabled the hobbyist or specialization aspect of it.Trac II came out in the mid- to late 70s. I think the switch to Trac II was pretty quick. Younger guys who were just coming into shaving age at that time never tried DEs. Some of the older guys who had been using DEs for years kept on using them, but very few of the younger generation started with DEs once the Trac II was available.
Note that this is just my recollection of what happened. I am 58 and never shaved with a DE at all. It was Trac II from the outset, and switched to Atra when it became available a few years later (I want to say maybe 1977 or 1978?).