The line varies. But it is generally generational, where it intersects with culture.
Even in the West, generational divisions (Baby Boomers, Generation X, Millennials) are fairly ingrained. These come from common temporo-cultural reference points, where they have intersected with the influence of these factors into daily life.
Think of the introduction of the motor car, domestic electricity, television and mass media, and the Internet - all life-changing technologies that caused huge divergence in lifestyle. Differences in morality, in politics; these also influence the way people speak.
In the linguistic ecology of Chinese topolect-speaking people, similarly large changes have occurred. But they can vary, and I would say there is much less sociological study asking how and why the variation happens.
A specific example: the 《福清方言研究》 published in 1993 explicitly states:
老派人数少，一般是七十岁以上的老人。新派人数多，一般是六十岁以下的人。六十至七十之间的人有的是老派口音，有的是新派口音，有的兼而有之，很难用具体的岁数作为分界。
As defined by this study of the topolect of Fuqing, in Fujian, these 老派ers would have been born in the 1920s or before, whereas the 新派ers in the 1930s or after. What that local change corresponds to, I am not entirely sure (I'd imagine it was to do with early 20th century urbanisation, mass migration, or possibly the introduction of Mandarin-based education, but I don't know the history well enough).
