I like to smoke whole briskets weighing between
twelve to fifteen pounds. I trim the fat cap down to about a 1\4" thickness all over. I also remove some excess fat between the point and flat. I then smoke it for around twelve to fifteen hours with the fat side up at 225. Most of the fat renders off while keeping the brisket moist. I've never run into a really aged brisket. My guess is that as you trim down the fat cap the fat will get lighter as the aged portion is cut off.
Good luck