What is the minimum work visa age in Japan?

What is the minimum age to work visa in japan

I am not sure that there is a minimal age for work visa, but there are a lot of other requirements for work visas in Japan: http://www.mofa.go.jp/j_info/visit/visa/long/index.html

For many of the visas you need completed university studies, or work experience and your work experience and your work in Japan needs to fall in one of the categories for which visas are available. I.e. there is not one "work visa" but a different one for each occupation and there are many occupations for which there are no visas at all.

I would guess 20 as that is when you stop being a minor in Japan, but as LikeBike says, there are a whole lot of other requirements. But there are categories of working visa (there isn't simply one) where younger than 20 would/could be appropriate.

(disclaimer - that is simply a guess based on a piece of logic)

working holiday visa
Be aged between eighteen (18) and thirty (30) years

Working visa
Be aged over eighteen (18)