There is no question whatsoever that if the age limit were increased to 17 and below, the marketing restrictions would be more effective in public health outcomes.
I'm somewhat perplexed by people mentioning that there's some legal threshold. As you said, the most typical legal threshold is the one that is set at 18 or 19 for alcohol. That was the threshold used for tobacco marketing and other types of marketing, so we need to be unequivocal about the fact that if this threshold covered marketing, it would not only be more effective for youth but would also restrict ads to kids, who inevitably see what's marketed to youth.
I don't understand when people refer to the legal barrier there, because we already have legal precedents in other domains.