Before exposure to Western medicine, the Japanese relied heavily on Chinese medicine and therapeutic techniques. Think acupuncture, herbal tinctures and pastes, bones and organs of animals, and an Eastern equivalent of the bile and blood "Humors" of Medieval Europe. The efficacy of these treatments is fairly low, for example aristolochic acid is common in several treatments including wild ginger and is found to cause a wide range of health problems; UTIs, various cancers, and even kidney failure with only moderate use. In the case of severe illness, Chinese medicine does not perform nearly as well as Western medicine, and as we see can even cause fatal problems.
The first Employee health insurance plan didn't grace Japan until 1927, so even with awareness of Western medicine in the country, access to it was probably not widespread. Life expectancy in the 1950s in Japan might be considered abysmal when compared to today's.
The reformation of the public health system following Japan's defeat in WWII is probably the biggest fish you're looking for, but unfortunately I don't know much about that, although this book might be of interest in your studies. The biggest scandals in public health in the first half or two-thirds of the 1900s would have been Itai-Itai, Minamata, and Yokkaichi asthma (caused by poisonous ash from a factory, shaved off five years from life expectancy, on average), all caused by the pollution of several factories into rivers over a period of about seventy years. After the last one, it seems the government clamped down more tightly on industrial waste, but again, I can't speak to that.
So, in the 40s in Japan, 50 was about the average life expectancy, but in the 50s an amazing leap in health involving new health regulation occurred which expanded a person's life 20 years, a 40% jump.
As you can tell, I'm no professor, but this is what I've gleaned from Google Scholar and some things my professors have mentioned. Please correct me, anyone who can.
The question might not be the right one to ask as the life expectancy may be a result of massive fraud on the part of Japanese citizens. In the linked article it states that over 230,000 Japanese over the age of 100 are currently 'missing'. It may not be that the Japanese actually have one of the highest life expediencies, but instead just have horrible record keeping.
So prior to western medicine adaptation, their lifespans were pretty normal for humans in that time period? Also, I've heard that the Japanese maritime diet accounts for at least some of their longevity.
Has Japan's life expectancy always been amongst the highest in the world?
You can see a chart over time of Japan's life expectancy numbers in relation to the rest of the world [alt link]. I made the x axis total fertility (birth rate) and dot size is GDP/person PPP.
When and why did it become so high?
Japan has always had above average life expectancy since at least 1865 (when the data on Gapminder starts for Japan) [source Riley File]. But it was not until post-WWII that Japan's life expectancy started to really take off. The question of WHY did it happen is much more complicated and speculative. I can only list things I have read about before, I don't have any solid conclusions other than that there must be many factors:
Most Americans know about the "baby boom" in post-WWII which shifted the birth rate from 2.8 to 3.7 babies per woman (1946-1960) before starting to fall to its current numbers. During that same period, the birth rate in Japan went from 3.4 to 2 and kept falling.
However if you look at the late 1800s during the Meiji Restoration, you can see there is a pretty significant drop in birth rate in Japan with no change in life expectancy. Then later up until WWII life expectancy in Japan did rise, but was outpaced by Argentina, USA, Canada, and most Eastern and Western European countries [note again that life expectancy data doesn't start until 1865]. So this data does not support 801_chan's theory about Western vs Eastern medicine, as Western medicine had been spreading in Japan long before the Meiji Restoration period with Dutch and German Missionaries.
There's also the issue of diet. Although I know a lot about the Japanese diet, I don't know enough about the science of diet vs longevity to comment further. But you should know that the people on the island of Okinawa have a pretty different diet compared to a typical Japanese person, and Okinawans live longer on average compared to Japanese. So diet alone can't offer a full explanation.
Besides quality, there's also the issue of quantity of diet. Some Asian cultures have long had a notion that eating less is a desirable thing (Hara hachi bu). [I feel that this is at least as important as quality, but this is outside the scope of this subreddit]
Sources and raw data for the above Gapminder chart can be find by clicking the three "various sources" buttons.
Japan specific sources for life expectancy data:
