The graph is generally showing percentage of the population having their first sexual experience (however that's defined) at roughly what age.
Y-axis is percentage of male or female respondents.
X-axis is year for which the numbers are relevant.
Triangle/Circle/Square refers to their first experience occurring at middle/high/college schools (which, if memory serves, is approximately 12-15, 15-18, 18-22 years of age in Japan).
Solid lines are male, dotted lines are female.
Taken as such, the statement that the overall percentage is declining is valid for the last 15 years or so (the one on the left), but the one on the right (and conclusion beneath) is inconclusive because it lacks data for the post-college age bracket.
Also, I think the note under the graph is incorrect, as that graph makes absolutely no sense if it was indicating a shift in rate in itself. It can be used to show the shift in rate, and to aid in explaining a shift in rate, but it's not a graph of the shift in rate.