If I would rule my country (not the world, if I would rule the world, people will live in tribal communities occupying city-states or wandering like nomads, worshipping pantheistic God and living from hunting, fishing, foraging, farming, sheeping/shepherding [is there such a word?] or trading)...
Drinking: Depending on age in certain amounts. Let's say begin at age of 13, allowed amount is minimal, then it increases according to age until certain limit.
Smoking: Same as above.
Sex: Either from 16, or depending on lenght / size of your sexual organs.
Voting: If I ruled this country, there wouldn't be democracy, I would be the ultimate emperor, the head of the only official religious cult and leader of military, politics and economy.
Tax paying: Always, but only when you work. So simply at the age you start working.