about low changed

Now in Japan, over 20 have responsibility without their parents.　Should the age of adulthood be changed in Japan from 20 to 18? I think depend on things. About drinking and smoking, I can not agree but, about murder or crime I must agree this odder.
First of all, drinking and smoking, it is legal to over 20 but, in Japan under age can buy both of them very easily. Drinking and smoking are bad health for everyone but under age person get more damage to growth. People going to be grow up until 20 years old, scientist said. If Japanese low changed adulthood from 20 to 18, I think more people think easy things to drinking and smoking.
Second, today in Japan is getting big amount of younger offender because, they don’t have responsibility and light punishment to under age. I don’t think if low changed adulthood from 18 to 20, decrease number of murder or crime. But , I think getting more people have responsibility by them selves.
I think if low changed adulthood from 20 to 18, they can get drunk and smoking very easily and after getting crime but, they should have responsibility. Responsibility is big things to be an adult. Today in Japan, under age need have more responsibility.