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
September 15 Birth day Kakashi Academy Grad. Age 5 Chūnin Prom. Age 6
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.
at the time of pain he was 30 so one year has passed in the forth great ninja war so he is 31. mr.dimitri mx you were so so close by one year. and also have we considered that kakashi died by pain and was revived so technically he restarted his life when pain revived him so isnt he really one years old.