I suspect the trough in the early 50's is the overlap of a number of factors. - kids are too old to be much fun anyore, but not too old to still be very expensive - career potential has peaked, age begins to become a liability - income has peaked and may begin downslide, yet expenses are still on rise (gotta pay down that mortage and those college tuitions) - health has peaked, mortality looms, your best years are behind you - kanoodling is limited to Christmas and birthdays.
Another way to look at it, rather than one trough, is as two peaks. Young: full of energy, plans and potential. Old: few responsibilities, effort has balanced ability. In between: energy and hopes waning while pressure of work rising. And even another way: Young: you're the recipient of help and support (from parents, etc.). Old: you're the recepient of help and supprt (from children, etc.). In-between: you're the source of help and support (for both parents and children).
When i was in my teens i knew for sure it was time for me to leave home cause mom gifted me wit a suitcase to pack my clothes an a watch so i woudnt miss my bus Please Register or Log in to view the hidden image!