I think it depends on the individual and their family. If that person is in a high risk group (Parents, aunts, uncles, grandparents are alcoholics and/or addicts) Those children should be warned of the consequences. Because usually those children have grown up watching their parents, aunts, uncles, etc etc.. get drunk and/or high and they are used to it. There is a thing called addictive behaviour. THat means a person has a tendency to become addicted to many things very easily and fast. Also an addictive behaviour means, to steal, lie, hide things and the list goes on... Those children should NOT be allowed to taste or try drugs and alcohol until they are old enough to understand the consequences.
Children who have a good upbringing and parents who actually talks to their children and give them lots of love. The children will be in a low risk group and less likely to become addicts. It is very very important for parents to explain to their children how they should not abuse things like that.
For drugs, I don't think children should try drugs at all until they are adults themselves. One puff of a joint, they will be high. One sip of beer or wine, they won't get drunk. They will know how it tastes, that's all.