Your argument makes no sense, because you're assuming the 18-20yo demographic is voting entirely differently than the 20-25yo demographic, which is obviously a stupid way of thinking when it's stated like that. Those groups are likely going to vote almost exactly the same, so extending the voting age downwards simply gives the younger voting block more power because it gives them more numbers. Also, the idea that 70% of them don't vote is just something you pulled out of your ass, probably based on American voting patterns. Japan isn't America, and assumptions like that are invalid in any argument.