I think your comment may have finally changed my views on the subject matter at hand. We should eliminate the taxes on cigarettes, and remove the red tape barring the simple pleasure from humans just because they haven't completed enough revolutions around Sol yet.
In fact, a truly meritocratic society would remove any such barriers. What difference does it make to have a 16 year old pilot a 2 ton general-purpose land vehicle vs an 18 year old? Shouldn't we just give them both a test whenever they personally choose to exercise that freedom, or, better yet, allow our citizens to use their personal property freely at any age so long as they are not intentionally harming others? Surely being able to acquire the down payment for a 2010 Toyota Corolla is the only mark of skill that matters to a thrifty and entrepreneurial society?
And it would be wrong of me with all this talk of individuals to not speak of my favorite Citizens United—NGOs! If adding so much sugar to a can of coke that phosphoric acid is required as an additive (to suppress the natural gag reflex) to increase market penetration 3% in the 4th quarter, then wouldn't the board at such a NGO have an obligation to their shareholders to deliver that growth?
Especially given that children and adolescents become lifelong customers, wouldn't such a board in fact have a fiduciary responsibility to target such demographics broadly with ever-sweeter processed foodstuffs and beverages? It's not the board's fault humans under the age of 25 generally don't have a fully developed pre-frontal cortex that could resist extraneous activations of the ventral tegmental area. It's just good business sense—if not us then someone else would clearly profit.
Or maybe we could tax the externalities, dunno. It seems your argument is no government could capture every single externality at any one given point in time and thus the effort is moot. We should just pick up the tab on our strained healthcare systems and add more motorized shopping carts at Walmart, and heck maybe we'll even get those cool floating chairs from Wall-E if Buy N Large can finally circumvent silly legalisms and deliver to the American public a sugar so concentrated you can skin-pop it.