If people are engaged in something that can (or is likely to) cause harm to others, then I make it my business, and it should be the business of big brother. I don't think a 95 year old, half blind grandma should be driving a car. That could endanger others. The same can be said for mentally disturbed people and guns.
Now, if someone wants to smoke weed, get married to someone of the same sex, read the bible every day, or drink battery acid, I really don't care. They are not causing harm to others, and it therefor should be no business of mine or the governments.