I agree it was sometime in the late 70s.
I think it was related to the rise of the Florida circuit. Before that, there were practically no shows after MSG (early November) until well into January, so it didn’t really matter when the new season started, and there was plenty of time to count points for the HOTY.
But once the Florida circuit got big, it was really difficult to get all the December show results processed in time for the Annual Meeting/Awards.
So they changed the competition year from “Jan 1 - Jan 1” to “Dec 1 to Dec 1”. That way the December results from Florida didn’t need to be processed until later, and they had all of December to get the HOTY points counted. (Remember, this was being done by hand, not on computers)
Once they changed the competition year, they pretty much had to change the age definition too. Otherwise you would have riders who competed a Juniors for December, and as Seniors for the rest of the year.