Public pensions are extremely low (~5600$/year). They are more a "survival minimal amount" for the elderly than a real retirement pension.
There is also a phenomenon of pride among the elderlies in Japan: It is almost unthinkable for them to ask money to their children (At the opposite, doing so is pretty common in China or Korea).
Considering this context and life conditions, it is no surprise that some elderly with the lowest pensions choose to turn into crime to try to overcome their situation.
Their is a lesson the west should take here: Western countries birthrate follows the direction of Japan ones. Considering our pension schemes will become also at risk, we might encounter exactly the same situation in some decades.