Traditionally prices for second hand vehicles go up as the young teenagers from back in the day, reach 50-60 yo.
At that age many people finally get on top of their finances. (Kids/mortgage gone. Near the top of their career.) And they are desperately reliving their youth. (Apparently
)
So, very approximately, 50-60 minus 12yo = '68 - '78 models should be peaking now.
I'd love a 1973 Z900 in the lounge room. When I was 12yo I knew they were the "best bike", and would be forever.
Unlike my "old" mates, I have no soft spot for British bikes. (Not as fast as a Z900. LOL)