The Collins dictionary says a generation is the average time between 2 generations of a species, about 35 years for humans.
It's not the English definition of "generation." but the Greek definition of "genea."
https://www.google.com/search?q=genea+definition+matt+24%3
WTS' previous definition (the wicked) seems to have been the most accepted.
The switch to "the anointed," and then "the anointed prior to 1992" is whack.