Conscription in Japan had started with the advent of the new National army after the Boshin war and the Meji restoration. It had always been a contentious issue among both the public who resented it and the old school aristocracy who preferred the old ways and saw conscription as giving military privileges to peasants. How conscription generally worked in the Japanese military was that there was peacetime divisions that were not at full strength, they were brought to full strength by conscripts who would be called up upon mobilization. There were 4 categories: Active service, first reserve (filled the divisions when mobilization happened), second reserve(helped with logistics and non fighting jobs, would only fight when necessary), and national militia(Home defense). Candidates were also graded based on physical traits. You could receive a grade of A-E, most soldiers came from the A or B grades as they were the most physically fit.

In 1943, a number of reforms were made to help expand the amount of men who could be drafted. This was done to help replace the numerous losses Japan was taking. The draft age was lowered to 19 (it was 20), it made Koreans eligible for conscription (though they were almost always assigned to labour battalions), extended the years one had to serve to five, and ended student deferment (students in University could use that as a reason to get out of the draft).

Source:

Japan's Imperial Army by Edward Drea.