The conclusion concerns adults’ smoking. Whether teens have also seen a decline in smoking doesn’t affect adults’ smoking.

The conclusion is not about the entire adult population. The conclusion simply asserts a causal relationship between restrictions on advertising and a decline in smoking. This doesn’t mean every adult stopped smoking or that every adult is affected by restrictions on advertising.

The conclusion is not about “today’s adults.” It assert that restrictions have “had” a significant impact on adults’ smoking. Evidence of what has happened in the past is relevant to a claim about what effects restrictions have “had.”

It’s not clear what impact restrictions of other products could have on smoking. The author did not assume that there weren’t restrictions on canned food, toys, or pencils, for example.

This possibility, if true, shows why the conclusion doesn’t follow from the premises. If other factors could have contributed to the decline in smoking, then the correlation between restrictions and the decline does not have to be significantly due to those restrictions.