You're reading too much into something, that's plainly obvious otherwise. DPC's nationality has nothing to do with the game setting, which is obviously the US, as the most familiar globally (unfortunately). The reference is obviously to the legal drinking age, regardless if MC drinks even before then or not.
As for your theory, it doesn't hold at all, since a 2 year time jump would mean we either have completely no control over the narrative, or the relationships just freeze for a couple of years. Either way is going to be extremely bad for an interactive game.