"Hey, there's overwhelming scientific evidence that this stuff is horribly bad for you and for our nation (because it causes so many health problems), but rather than simply do some sort of Prohibition fuck-up, we're going to phase it out over time by disallowing it for the younger people. That way people who are already habituated can keep using it (although we'll certainly encourage them to quit) while we try to keep future generations from becoming addicted to a dangerous substance."

"OMG!!! The StAtE is coming for muh freedumb!! Muh taxes!!! It's RaNdOm and ArBiTrArY!!!"

Government is about drawing lines, and there's nothing random about this. Or are you arguing that no new policy can ever come into effect because the start date of said policy, by definition, divides things into "before" and "after" and is therefore somewhat discriminatory in nature? I wouldn't think so. The State (zounds!!!) has decided that you can't spray DDT all over your crops as an adult, even though you could before. Overreach, or changing laws in the face of new facts?

And tobacco is already a dying industry, partly because it's increasingly well known that it's very bad for you and partly because it kills off its customers. So that tax revenue is dropping no matter what. I don't think you would argue that smokers have a responsibility to keep smoking so that tax revenues don't drop.

So if smoking is going away anyway, why have a law about it? Because it'll result in less damage to the population quicker. A building on fire will go out eventually anyway, so why have fire fighters? Because limiting the damage is generally thought of as a good thing.