Board Thread:Legitimate Canon Questions/@comment-94.195.111.213-20150220212821/@comment-4091815-20150220213314

According to her grave stone - Kate was born in 1983 meaning she was 27 or 28 at the time of her "death" in Season 1.

It is currently 2012 in the canon meaning she is 28 or 29 when she reappears in Season 4.

We have nothing canonical by which to determine Derek's age.