Well, I would say it's a legal standard for a reason. Either one accepts that somebody in this situation and at that age has impaired judgement (like under the influence of alcohol would be), then it's a technical non-con since John knows Dean's age and situation. Or you don't think that a 15 year old is impaired then it's not a non-con. One could argue that labelling underage or giving Dean's age makes a non-con label redundant and it is implied, but then why bother to label as dub?

I just really don't see how the dub enters here. John knows how old Dean, Dean knows how old he is, it doesn't seem like there is a spell that makes him more or less mature than usually.

As for it varying from place to place, well, there might be cases where it is complicated, but since SPN is set in the current US, is an US show and probably has a large US audience, why wouldn't one pick US standard?