I think this "time lag" happens because people fix their ideas about older generations when they are fairly young and don't revisit their preconceptions as they age.
I do this to some extent. When I think of people turning 40, I think of my mother's 40th birthday, not my friends. When I think of the older generation, I think of my grandparents, but my remaining grandmother, the youngest of them all, is 90 this year and was only in her early teens when WWII broke out. My grandfathers that fought in the war are now all dead; my late paternal grandfather would have been 110 this year.
In short, my references were probably set sometime in my teens, probably around the time I was 18-ish. Old people were my grandparents (the war generation), middle-aged people were my parents, young people were my generation, babies were my little cousins.
However, I do think that one reason for the general impression that previous generations were less tolerant, open-minded etc than newer generations is that we have seen a kind of "progressive metanarrative" take hold over historical consciousness in the last sixty years. This metanarrative now infiltrates almost all social, cultural and political thinking in Britain. A perfect example of this is the choice of the M People song "Things can only get better" for the 1997 Labour GE campaign.
The problem with this metanarrative is that for it to be valid and have power, previous eras need to be perceived as worse in various social, cultural, economic and political terms. And believe me, there has been and still is an underlying drive to portray previous eras in a negative manner.
I notice it most particularly when it comes to women's issues. On both sides of the political spectrum, there is this pervasive notion that prior to about 1975, married women simply did not work and that married women once they had children were not economically active at all for the rest of their lives. There is also this vague suggestion that females, full stop, didn't really work before they were married either.
Connected to this is the idea that women married in their late teens and early twenties, that no couples cohabited, that all couples got married in a church, that illegitimate children were few and far between, that female gang violence is a modern phenomenon ... I could go on. Incidentally, there is also a belief that formula milk is a modern invention.
All of this is utter bollocks, but both the left and right perpetuate this revisionism: the left so they can convey the idea that their ideology rescued women from these confines, and the right to describe a utopia where society was prosperous and stable, and to which we should return.
One thing I think is rather fascinating to consider in light of this thread is that Jimi Hendrix would be 72 if he were still alive. 
Peppermint ... I am afraid to say that quotation appears to be a twentieth century invention. It is attributed to a variety of classical authors: Socrates, Plato, Cicero, Livy, Hesiod, Peter the Hermit ... well, the first problem is that Socrates left no written work, so he is out of the running from the off. 