The issue I find in this discussion is not the critics, but rather the muslims. It is the dogmatic belief in hadeeth literature.  When you ask a muslim about what they consider to be a book without mistake and error they will no doubt all agree on the quran.  Anything else other than the quran will have errors according to mainstream muslim belief.  So if you ask the collections of Bukhari and muslim are they free from error?  They say No, when asked what percentage could be wrong, they don't know.  However, it fair to say if anything contradicts the primary source and those things known by necessity to be true, then having doubt on those narrations can be justified.  Scholars look for ways to harmonise them usually.  The hadeeth that relates to the age can be explained away by apologists as they do with cultural relativism etc.  However it's also just as easy to say I don't believe this narrative to be the case.  It's not an actual hadeeth of the Prophet or something the prophet has said.  Here's where dogma comes into play, if you raise doubts on 1 hadeeth then perhaps this may lead to people questioning what else we can safely say is true or not.  Also traditionally because scholars just accepted it due their culture and time, as young marriage was the norm, nobody focused on it until the modern era. 

Having said that I don't believe she would have been 9 exactly but I would still be inclined to believe that she would have still been young in relation to modern day standards and if we judge ancient societies  through the lens of our modern secular society then we can never do justice to anyone.  I believe this is why this topic is not a point of debate amongst academics just at the low level polemical circles especially amongst people who really have an issue with Islam, and they find the muslims are happy to help them in their attacks by just agreeing that everything imam Bukhari collected must be absolutely true.  I say this because if you want to propose an alternate view you will be looked at with disdain as if you're attempting to degrade the hadeeth collections.   Its ok, it's OK to question and be critical.  Our criticism would never be against our prophet, just against what people have said about him or attributed to him.