Yes, some of the explanation is an increased population, but it's also clear they're committing more crime than they did.
The graph in the article (1990-2016) looks like the proportion of offences committed by pensioners has roughly risen from 3% to 20% (6.6x increase, maybe 5x if it's 4% to 20%), yet the proportion of over 65s (1990-2016) in Japan has risen from 11.9% to 26.6% (a 2.2x increase)[2].
The source of the BBC graph is in Japanese[3], so I can't get exact figures for the crime and papers I found that did have exact figures only went back as far as 2005.
The graph in the article (1990-2016) looks like the proportion of offences committed by pensioners has roughly risen from 3% to 20% (6.6x increase, maybe 5x if it's 4% to 20%), yet the proportion of over 65s (1990-2016) in Japan has risen from 11.9% to 26.6% (a 2.2x increase)[2].
The source of the BBC graph is in Japanese[3], so I can't get exact figures for the crime and papers I found that did have exact figures only went back as far as 2005.
[1] https://ichef.bbci.co.uk/news/624/cpsprodpb/B06F/production/...
[2] https://data.worldbank.org/indicator/SP.POP.65UP.TO.ZS?locat...
[3] http://www.moj.go.jp/housouken/houso_2008_index.html