2004 is most definitely not "aged." It troubles me that so many online retailers are starting their "aged" sections at years 2004, 2003, 2002. I don't think anybody really considers these ages to be "aged." Aging takes place slowly, and all the time, so there isn't "not-aged" and "aged" tea, there are stages all between. I can notice differences at early as one year, some change notably within 6 month periods, depending on storage.
There is also a 5-year rule. This means that puerh goes through stages in 5-year intervals. Not true for all cases, but is a good general rule. I've noted that most 5-years are a lot like fresh ones, but have lost most obnoxious character. Like a baby who finally stopped crying.
10-year range is what I call adolescent, and I generally find them to taste acidic, complex, and quite tasty (most of what I have in this range has been wet-stored 7-8 year).
15-year is "aged" to me. 15-year has much more of a mud/matured lumber characteristic, and a much more developed chaqi. I don't have any opinions from there on, as I've tasted even less of this older puerh.