The idea of using "belly sag" as a criterion for aging bucks is WAY overblown, in my opinion. In the summer, use chest depth as the best judge of age. And using that criterion, that buck is 2 1/2 or 3 1/2 at best (middle-aged).
You can't use the size of the neck for aging in summer. All bucks have narrow necks at this time, even mature bucks. They don't fill out their neck muscles until mid to late October.