Unfortunately higher insurance premiums don't actually wipe away the negative externalities caused by you smoking. If I didn't have to pay at all for the costs of healthcare for the obese, them saying to butt out would likewise make a lot more sense, but
US taxpayers directly fund more than two-thirds of health care. That makes it yours and my and everyone else's problem.
That's the whole problem with the concept of rugged individualism and personal liberty over everything else. It's fundamentally incompatible with being part of a modern society. The reality is we're all paying to one degree or another for others' bad choices.
I agree that there is no real bright line between childhood and adulthood, immaturity and maturity, but that's something no law will ever be able to legislate. That doesn't mean it's not valuable to have those lines drawn (age of consent, et al.)