I don’t know if it still happens, but two years ago I was still seeing lots of job ads clearly saying “35 years old or younger”. The main reason is that the company will have to pay an increment for age and possibly having dependants, as described in the post before last. All this extra outlay would come without any guarantee of being a better employee than someone straight out of college, as job references don’t exist and people mainly leave good companies for bad reasons.
Japanese companies also seem to prefer employing “blank slate” graduates who they can then train into the company way and slot into the company wherever they like, graduate recruits not being given any hint of future job title. Actually, train is probably the wrong word as, just like Japanese society generally, new employees are supposed to pick up what to do by example and other forms of osmosis rather than actually being told anything. This is obviously even trickier for older people to manage. There is also the problem that seniority gets confusing when age doesn’t match with number of years in the company, as it would if everyone was recruited straight out of university. In another post I came up with the very hesitant theory that maybe Japanese men usually dye their grey hair as they’d be embarrassed to be thought of as old and still in a lowly position in the company. There was a funny sketch in the excellent show Salaryman Neo where a new employee kept on being mistaken for a manager in this way, and most Japanese men I know with natural grey hair are in more creative jobs.