The vast majority of people who smoke start before they turn 18 meaning it's already illegal to buy tobacco products when most establish the habit.
Drug prohibition simply doesn't work. Never has, never will. All it will do is criminalise users and create black markets that will take the sales revenue out of the pocket of the government and legitimate businesses and put it in the hands of criminal gangs.
If criminalising users stopped them using there would be no drug users which clearly isn't the case. Instead it makes addicts less likely to seek treatment and forces them to deal with shady characters to obtain their supply rather than buying from regulated sellers.
We should be looking at the evidence from the other countries and be legalising cannabis and decriminalising use of other drugs but instead we're going down the regressive authoritarian route of trying to ban everything. Tobacco use in the UK has already reduced massively over the last few decades thanks to public health campaigns, heavy regulation on advertising/packaging/media depictions and the high duty. That's the route we should keep pushing.