All that means is that Japanese young people are getting married later than was customary 30 years ago. For the eldest son, it's still expected in some families that he will never move out, even after getting married (my brother-in-law was roped into this, after having moved away to Kanagawa. He gave up his job and moved back into his parents home because his father more-or-less ordered him to). It's cultural as much as economic.
Maybe so, but it's not a sign of excess wealth. Japanese young people live with their parents longer than was customary thirty years ago, so it's not a question of culture. They simply can't afford to move out.