Depending on the type of wine, young can mean one of two things. If a wine—such as Beaujolais Nouveau—isn't supposed to be aged, then young could be a compliment, much like fresh or youthful. Some Rieslings are best enjoyed while still young, before they lose their pleasant crispness and floral perfume. If, on the other hand, we're talking about the latest vintage of Dows Vintage port, a wine that will definitely improve with age, then "young" would imply that it's immature and still needs to develop in the bottle.