My point is that "consenting adults" aren't really involved here. A "think of the children argument" is when you argue for banning something that adults do because a child
might be exposed to it and
might suffer harm as a result. An example would be wanting to ban porn (something for adults) because a little kid might see it and warp their fragile young mind, or whatever. The key part of this scenario is that the little kid's exposure to porn is neither intentional nor desired (on the part of porn producers) or common.
Starting smoking, on the other hand, is something that, for the most part, only children do. The entire goal of tobacco companies is to get children addicted so they'll keep buying nicotine for the rest of their lives, without any adult decision-making involved. Here are some numbers for you:
Almost 90% of people in the US who smoke tried their first cigarette before they were 18[1]. Effectively all of the rest did before they were 26 (most by 22), which is around the age where the human brain's capacity for risk assessment and long-term decision-making is fully developed. Two thirds of daily smokers started doing that by the time they were 18, and over 95% did by the time they were 26. Over a quarter of daily smokers started before they were 15. Furthermore, people who start smoking are most likely to do so with the help of their peers (i.e. other children), not adults.
The concept of rational free choice is a bit murky even under the best circumstances. When you add in addiction, it gets a lot more complicated. And when you're talking about children getting addicted, I don't think it's a helpful framework. Children are not capable of consenting to a destructive long-term addiction, and in practice, almost nobody else does.
[1] https://www.ncbi.nlm.nih.gov/books/n/surgtobyouth/pdf/ See chapter 3, especially Table 3.2.