This is based on the crude road injury rate (all injuries), and a reconstructed population up to 90 for England and Scotland from 1985 to the end of 2014. The colors show the injury rate (per capita), for all police reported road injuries. It’s very clear that “around 20 year olds” have the highest per capita injury rate. Some dotted lines have been added for particular cohorts. It is possible to persuade yourself there are cohort effects.
It bothers me a little in fitting an Age-Period-Cohort model that this impression will be reinforced by assumptions as much as by data.
When you subset the data a bit, there are some very clear cohort effects, for example for motorcycle injuries.