I have seen multiple children that have been brought up in a conservative household. As these children turned 18 they consumed copious amounts of alcohol and in quite a few cases passed out! Meanwhile the children who where introduced to alcohol at an early adolescent stage new how much alcohol is a suitable amount and did not drink as much as the people from conservative households .
There are certain instances in which underage children may be exposed to alcohol. For example, flavorings used in food (like vanilla, almond extract, etc.) typically contain alcohol. Some may partake in communion. However, in general, children should not be allowed to ingest alcohol. Even minute exposure to it could cause them to believe it's okay to imbibe, and children do not have the tolerance for alcohol that adults do.
There is no supporting reason to provide children with alcohol, so there is no reason to give kids small doses. That said, people who are old enough to sacrifice their lives for the country should be able to drink (as well as gamble) as they please, so the legal drinking age should be lowered to 18, or the legal age to get into the military should be raised to 21.