I think the notion that human populations exist in generations is fallacious because it suffers from a subjective bias. Any individual who makes a claim contingent on the idea of a generation automatically thinks in terms of the distinction between his age group, his parents' age group and his offspring's age group without realising that he is not a discrete reference of existence. There will always be an equivalent set of relations one year younger and older up to and exceeding the adjacent generations. If that is true then the notion of a generation is meaningless.
What then is meant by such references as 'Generation X'?
To my mind, these are markers of sociocultural phenomena or shocks that are so distinct that they captivate a large band of ages who then are associated with that shock. For example, the world wars of the 20th century affected many people in the same way so that large age groups responded in similar ways leading to large levels of social congruence in those populations. Similarly, technological phenomena such as the emergence of computers and mobile telephony have acted as 'age regressors' for age groups - batching otherwise disimilar individuals to guide their actions. Therefore, it is the chronological markers of these phenomena which define the generations rather than ages per se of the individuals.