I do not think that a language, even if not "widespread", should not be taken in consideration in terms of "oldness" (sorry for my phrasing)... I've seen someone who quoted Max Weinreich "a shprakh iz a dialekt mit an armey un flot" (= a language is a dialect with an army and a navy), and I think it is pretty much the same with Chinese ethnical minorities's dialects. Now, I would not be able to say which of them are the oldest. As for the diverse languages among the Han-group, I do not know either. Need some research...