I believe the correct interpretation is: They ran the study periodically from 2000 to 2018. At each point, they surveyed groups, of which, one bracket was "me

I believe the correct interpretation is: They ran the study periodically from 2000 to 2018. At each point, they surveyed groups, of which, one bracket was "men aged 18 to 24". Within that bracket, one third of those surveyed reported no sexual activity in the past year. This was roughly consistent over the entire 18 year period.