6.5 sounds right for peak trophy size.

I remember watching a Realtree show where they showed a 177 and 189 class buck that was shoulder mounted and stated they were both 6.5 years old. He then proceeded to show the sheds they had collected from both of these bucks at 3.5, 4.5, and 5.5 years old. Each set was a shooter but each year they gained at least 15 total points. I'm not a trophy hunters but he did get his point across about passing them up until at least 5.