Lots of scriptures in the old and new testament defining what a scriptural generation is. The Society has chosen to completely ignore those and no longer lets God's word interpret itself on this, choosing vague and dubious secular and ad hoc definitions and fallacious 'special pleading'.
Speaking about the rebellious attitude of the nation of Israel who had came out of Egypt under Moses it says at Numbers 14:29: ‘All your registered ones, from 20 years old upward who complained (vs30) will not enter into the promised land (vs33). They will become shepherds in the wilderness forty years to answer for your acts’. Then at Numbers 32:13 it says ‘God made them wonder in the desert for forty years until all that generation that offended God came to their end.’ So, all that generation with a minimum age limit of 20 years came to its end from God’s stand point 40 years later. Jehovah held responsible for their actions all registered males of 20 years and above (Num 14:29). God referred to the full time limit of one generation as being 40 years (Num 32:13). Interestingly, God told Moses that the price of a vow made by a male person in the 40 prime years of their life, between the age of 20 to 60 was 50 shekels, the highest price (Lev 27:2-3, Deu 1:35, 39, 2:14-16, 32:5).
Moving to the Christian Greek Scriptures, Matthew chapter one lists 42 (14 + 14 + 14) generations from Abraham to Jesus (some debate over this – anywhere between 41-46 generations). “So all the generations from Abraham to David [are] fourteen generations; and from David until the carrying away into Babylon [are] fourteen generations; and from the carrying away into Babylon unto Christ [are] fourteen generations.”- Matthew 1:17