Here's another old but good thread:More on Wet Aging
I acutally have two 18 lb'ers briskets in the fridge. I asked the butcher for the kill dates on the boxes that they have (since these were already out). It's a risk, not knowing the actual date, but for me, when I have a date I'm not sure of, I'll still age them 30 days (figure they possibly could have been 2 weeks old, Sam's goes through too many for them to be older than that.
KEY thing about the date is the max date, how long you want to push the envelope. I see 21 days as "optimum" but most brisket agers tend to go longer (isn't longer better)
FYI, make sure it's in the original cryovac. You can NOT wet age something that's been cut and put in trays and rewrapped. Air has gotten to the meat surface and if you reseal it, that air carries goodies that will turn the meat bad when aged.