I've been wondering, since I can't find an exact age... While watching these filler episodes, he appears as the ANBU Kakashi, who was leading his team. Being the Senpai of Itachi he looks exactly (maybe for me) like he does in the present day... Is this a coincidence? Or couldn't the animators be bother to add additional "lines" to his face? Note that we've seen the newborn Naruto, that would mean that Kakashi looks the same as he did 17-18 years ago..
Using this timeline and the following data
We can tell that Kakashi was born 14 years before the 9-tails attack. Using the information that the Akatsuki dead line was 18 years after the 9-tails attack. we can tell that Kakashi should be roughly 32 years old.