The Watchtower's doctrine of the overlapping generation is based on a fallacy of equivocation. There are two different uses of the word "generation." One refers to all the children of a particular set of parents. If a couple continues to have children into middle age, you might have a situation where one sibling is 25 years older then the other. Nonetheless, in this use of the term, they are of the same "generation." If the older sibling has a child the same age as the younger one, the younger sibling and the niece/nephew would be part of different "generations" in this sense of the term, even though they are the same age, since the niece/nephew would actually be the grandchild of the original couple. This is the sense in which the word is used in Exodus 1:6, with reference to Joseph, his brothers and "all that generation."
However, there is another, somewhat looser use of the term. We may speak of, for example, the World War II generation, meaning all the people who were alive at the time of the war, irrespective of their position in any particular geneaology. When Jesus spoke of "this generation" in Matthew 24:34, it seems obvious to me that he was using the term in this latter sense. He was speaking to the people who were standing there listening to him, regardless of their age or ancestry. Those people, the ones alive right then, were the ones of whom some, at least, would live to see Jerusalem's destruction in the year 70 A.D. They might have been 10 years old when Jesus spoke, or they might have been 40. There might have been grandparents present with their grandchildren listening to Jesus. Nonetheless, they were part of "this generation" in the second sense of the term.
By conflating the two senses of the word, the Watchtower has very cleverly created a fallacious basis for its doctrine of an "overlapping generation" that is not obvious on first reading.