I am in my 20's and am generally clueless but I agree that a backpack is not good... but, I will also say that I have worn a backpack (
) that is kind of boring/sporty (looks like this one
) and I think no one really cares. This is just for getting around a conference when I also had to carry around another bag in my hands. I think that if you get a backpack, it's not so bad if you don't take it so seriously, such as the Saddleback bags which I think are a bad clash.
(I agree with braised in his post above, this is just another take on the same idea I think)