The garment is worn only by the unmarried as, historically, after they got hitched the girls had to start doing housework, cooking, cleaning and bringing up kids, so the seriously-get-in-your-way sleeves got the chop.
The scene in the open square in front of the hall made me think of an overstocked tank of tropical fish. Kimonos of every colour in the spectrum swooped, coalesced and dispersed. Squeals of delight at glass-breaking pitch erupted whenever friends were spotted in the throng.
It was a scene of shuffling steps, caused by the restrictive garments and the accompanying zori sandals, spectacular obi belts tied at the back in impossible knots that would challenge the most prepared scoutmaster, enough fur stoles - faux and not - to get a tribe of Eskimo through the winter and enough hairspray to make another ozone hole.
It was a big, big hair day with styles bordering on the beehive alongside the traditional Japanese style adorned with buds and florets. Long fake lashes and longer fake nails completed the look.
The kimono is such a complex item of clothing - even standard kimonos retail for around $10,000 - that very few women can put the three layers on unaided, leading to the emergence of professional dressers for whom seijin no hi is their busiest day, with bookings for the sometimes hour-long fitting beginning at 3am.
Hours go into the preparations, since this day is the first time most Japanese lasses wear adult clothes, so everything must be just right. Formal portraits often follow the festivities, once used by parents to showcase their daughters to possible suitors.
The young men have a far less stressful morning. A few were decked out in hakama, the baggy samurai pants that were, perhaps, MC Hammer's inspiration, but most were in the sombre suits they will soon be donning every morning as they start lives as overworked, over-drinking salary men.
Wandering through the crowd, the infamous Japanese shyness was nowhere to be found as everyone happily posed for the foreigner with the camera.
I asked one young lady what becoming 20 means to her. "I'm free! I'm free!" she squeaked.
The young men I asked invariably answered something like: "I can drink!"
Among the crowd were a few oriental flowers from the Asian mainland, part of Kobe's generations-old Korean community, in the traditional dresses called ham-bok.
There were even a few young ladies in oiran style, with off-the-shoulder kimono revealing glitter or fake tattoos. Oiran were the forerunners of the geisha, hired entertainers and ladies of the night. In still-conservative Japan, wearing the oiran style is tantamount to dressing as a hooker to go to the high-school ball. Somewhere in Kobe there was a mother thinking: "Why did she have to go dressed like that?"
The oddest and most alarming aspect of the day was the heavy police presence. The streets leading to the hall were lined with uniforms and three buses with metal-grilled windows waited to haul troublemakers to the cells for the night. In recent years all-out brawls have broken out between rival gangs of young men who fancy themselves as street-fighting comic heroes and who have welcomed their foray into adulthood with too much sake.
Such incidents have led many of the older generation to the opinion that these young adults aren't up to the challenges and responsibilities awaiting them. However, the senior folk in a very age-weighted population will have no choice but to rely on the flighty young upstarts: in 2012 the smallest number in recent history of Japanese reached their seijin no hi.
CHECKLIST
Getting there: Air New Zealand operates five services a week to Tokyo (Narita) and four services a week to Osaka (Kansai).
Ian D Robinson enjoyed the celebrations under his own steam.
