Previous generations always thing the next generation is not good enough. If you hear from someone that the youngs are now wilder and break rules, someone have said the same for them when they were younger. This have probably continied since ancient times, but there is no data prior the formulation of psichology science.
A generation is "all of the people born and living at about the same time, regarded collectively". It can also be described as, "the average period, generally considered to be about thirty years, during which children are born and grow up, become adults, and begin to have children of their own".