It should largely depend on who actually faces the consequences. Buying a bag of white sugar and eating it once a day is pretty much as close as you can get to an objectively stupid idea, but almost nobody supports banning it because if you don't like it, you can just... not do it. Butt-chugging a pint of everclear and driving down a public road is also quite stupid, and most people support banning that because there's no real way of getting around the risk of physical harm it causes. Even if you don't do it, you can be mulched by a driver who does on your daily commute.
There are cases where I think we can reasonably say the brain is unfit to make decisions. In a society where putting formaldehyde is legal, a person could theoretically just check the ingredients and avoid it every time... but the human brain hardly has the capacity to scrutinize every single product or service it uses for safety shit like that, for decades, cradle to grave. A desperately poor person could theoretically learn about interest theory and see why a payday loan will eat them alive, but they probably aren't in the frame of mind to do that. A child has underdeveloped planning and reasoning skills, and can't reasonably be expected to fully process the impact that taking up smoking will have on them in their 40s and 50s; thus, we should protect them from themselves.
