Found this article, it's way easier to understand than Watchtower's definition of a generation!
How Long Is A Generation?
In general we think of a generation being about 25 years - from the birth of a parent to the birth of a child. We also generally accept that the length of a generation in earlier periods of history was closer to 20 years when humans mated younger and life expectancies were shorter.
However, generation lengths are not certain and keep evolving. We are now already starting to consider that a generation could be longer than the accepted 25 years. Men could be at least a third longer, so 35-year generations with women one-sixth longer, 30-year generations.
Why Age Matters
In family history, the length of a generation can be a really helpful way to check your research. If you had a generation gap of say, 35 years or more, it may be worthwhile looking into this a bit further. What you could find is that you’ve missed a generation, or that information on two different individuals has been linked to the same person.
This type of discovery could then lead you on to finding a whole new branch of your family tree.
Because generation lengths help us to spot missing information in our family lineage, it is a good idea to decide what generation length suits you.
If you are looking back at ancestors 50 or more generations in the past, using the ‘accepted’ 20 or 25 years as a conversion factor could underestimate the time interval. But if you’re looking back only a few generations and you want to convert generations to years, using 30 to 35 year generation gaps, or even one that you’ve developed based on your own family history research, is a great place to start.
You could then check the accuracy of these generation periods by comparing all-male or all-female ranges in your own family lines for a chosen period, perhaps 1700 to 2000. Then you would see how closely the intervals agree with the number of years you are using to estimate a generation.
In family history, conclusions about relationships can change whenever better evidence is discovered. Similarly, current understandings are subject to change as more data becomes available and the understanding of this data improves. This is also the case with generation length; so don’t feel that you must follow the latest convention, using a length that you feel comfortable with is fine.
The length of a generation and using them as a tool to researching your family history is a great way of checking your findings as well as making an unexpected discovery; you never know what you might stumble upon!