Congrats on getting your bull, and for posting the follow up report. I've always said that bullet placement is most important, but I still wouldn't recommend a .300 BO as an elk cartridge.
As to the age of your bull, because of the length of the 5th and 6th points, I would think that this was his first set of 6 point antlers, making him 3.5 or 4.5 years old. Should be great eating.