I think first of all you need to reconsider boy ages and coming of age ages, particularly for the peasants. There would not be that clear line between boys and adults, particularly in a society were schooling was poor, non-existent or not affordable. This means that boys had to start contributing to the family work effort very soon for pure survival matters.

Life expectancy was not around 80 like in most developed countries today but closer to the 30-40 range.

Mild alcohol consumption, at least in Britain, was relatively normal for most members of the house in Medieval times. I am referring to ale, and yes it was consumed all day long and was part of their basic diet.

Last, but not least, there would not be a social sense of wrongdoing for selling a bottle of wine to a boy; if he showed up with the coins he would get it, whether the innkeeper considered if it was for self consumption or for the boy's parents is just speculation. However, in many places was not that different 40 years ago, as a child I could go to shop or the tavern around the corner and ask for a bottle of wine or a pack of 6. The shopkeeper would simply assumed, correctly in this case, that it was for my father. But I would still be able to do the transaction without obstacles.