Don't make the mistake to think these jobs are just there to keep bored retirees active. The basic old age pension (kokumin nenkin - for sole-proprietors and people who have worked in non-permanent positions or small companies) is significantly below the poverty line at 70,600¥ per month -- that's $450 and that's if you've worked for 40 years without any gaps.

The vast majority of old people working these jobs, e.g. construction site security, do so because they would likely starve without it.