Generational Compaction
Mike,
I don't think S&H necessarily invalidated their theory or the generational mechanism behind it at all. Their modification in T4T from what they stated in Generations was clearly spelled out.
They said that the youth phase contracted from apx. 25 years long to apx. 21 years long. They explained this by saying that the age that one is culturally seen as an autonomous adult dropped by a few years and they gave one explanation as the drop of physical/sexual maturation (to the tune of three years over the past two centuries) --and this HAS occurred, due to some yet-to-be fully understood environmental reasons presumably associated the post-agricultural world.
Though they left out any other reasons, I would add that Industrial society further lowered that age by emphasizing the nuclear family at the expense of the extended family, whereby young twentysomethings were not as likely to get support from older family members. There are perhaps other aspects of the Post-Agricultural world that also lend themselves to this phenomenon (ie., the shortening of the youth phase where the Saecular cycle is concerned).
If you stick with their analogy that the length of the youth phase sets the aproximate length of the other phases as the distance from solstice to equinox does the seasons, and presto, you have a shortened Saeculum.
Heck, kids seem to be getting older and older at younger and younger ages, if you know what I mean. I wouldn't be surprised if saecularly-speaking the phase length dropped to 20.
Who knows, maybe S&H are full of crap, maybe the Saeculum is just a very imaginative fiction. But I think they are really on to something and I see the generational mechanism they identified as the only thing that makes sense in terms of the self-perpetuating nature of it all.
Americans have had enough of glitz and roar . . Foreboding has deepened, and spiritual currents have darkened . . .
THE FOURTH TURNING IS AT HAND.
See T4T, p. 253.