https://pubmed.ncbi.nlm.nih.gov/28850065/

ASH surveys showed a rise in the prevalence of ever use of e-cigarettes from 7% (2016) to 11% (2017) but prevalence of regular use did not change remaining at 1%. In summary, surveys across the UK show a consistent pattern: most e-cigarette experimentation does not turn into regular use, and levels of regular use in young people who have never smoked remain very low.

Kids smoking are at an all time high and so is vaping. Raising the age limit didn't do anything to help reduce this, because kids haven't been allowed to smoke for decades now.

Also, this is literally in your link:

While it may be surprising that the new T21 law didn’t reduce cigarette smoking across all types of smoking behavior,** explanations include pre-existing declines in smoking nationwide, **enforcement challenges at the state level, increased use of other products (e-cigarettes and marijuana), definitions of smokers in the study, sales outside of retail stores and other tobacco control policies.

Crazy thought… people aren't smoking anymore. No wonder it's in decline…