Knowing how kids are, they will just snicker and skirt their way around these bans anyway thinking they are some super bad ass. This is mostly symbolic.
The data we have on bans on underage drinking and smoking show that they work. Some kids will still smoke and drink, but the number is reduced, drunk driving accidents go down, and eventually fewer adults abuse alcohol and smoke cigarettes.
The myth about age limits making it forbidden and attracting more kids to do it is just that it’s a myth. Spend some time looking at the studies. They almost universally show that age limits on drinking and smoking are harm reducing.