Japan is often portrayed as a modern utopia, but beneath its polished exterior, one of the world's oldest populations is reaching a breaking point. As traditional multi-generational families break down, millions of elderly people are being left in isolation and poverty - with many turning to petty theft to survive. As Japanese prisons become retirement homes for a growing number of elderly convicts, Krishnan Guru-Murthy investigates this geriatric crime wave gripping the country