Look, I know this is counterintuitive, but 1/1, 1/2, 1/4, and so on and so on. Euthanasia doesn’t solve the problem of each generation being half the size of the previous generation. You’d have half the workers you had previous. Even if you killed everyone at 65, you still will have a worker shortage.
A lot of the problem is that Japan is crowded and the price of real estate is high for the average home buyer. As the population shrinks this will reverse.
As far as a worker shortage, that isn't the problem. The problem is the ratio of workers to people being supported by the workers. If you kill off the old people, you no longer have to pay their pensions and health care.