The young or cool look doesn't seem to bother anyone for other currencies (e.g. Doge).
Other currencies don't have the unique marketing challenge that we face -- that is, overcoming the stigma of being the "Dark" coin, and all the connotations that arise from this in the mind of potential users & investors.
If that is truly there case, then we need to change the name. It is earlier enough we still could. If being a dark coin is going to be such a challenge to get mass adoption, what is the point of calling it darkcoin?
Personally? I don't agree with a word I just said, because I also don't think we should be running from the idea of "dark" like you do. I don't think however we market it should promote that first and foremost, but at the same time it shouldn't be ignored. You can be professional and edgy at the same time.
The more I think about it, number 2 is the best balanced. It's professional and edgy, but also looks very trustworthy, cutting edge, and lightweight.