The data [1] don't really show this. In particular as recently as 2001-2003 35% of young adults (in the US) said they smoked, versus an all-time historical high of 45% across all age groups for some context. By 2019 that figure was down to 10% for young adults. The first bans on advertising smoking went into effect in 1971!

So what happened in the ~2000 era window of time? The internet and social media started exploding in popularity. In fact you'll find that the graph of the decline in teen pregnancy [2] looks roughly similar to the decline in young adult smoking, and probably for the exact same reason.

[1] - https://news.gallup.com/poll/509720/cigarette-smoking-rate-s...

[2] - https://www.pewresearch.org/short-reads/2019/08/02/why-is-th...