Phil it's really only a guess, that buck could be a 3.5 just as well. I shot a 3.5 back in the day that scored 167". I worked on a mid150"s 6.5 today. I've seen 6.5 in the upper teens. Unless you have the teeth, it's a toss up.
Mass is one pretty good indicator of age though. At 4.5 they start getting thicker. Every 5.5 plus I've seen has mass, even those in the teens.