Since much of MN forests are Aspen/Popple that probably is reflected in that age class. When I worked for the MN-DNR Forestry the optimum age of Aspen to be cut was around 48 years depending on the site index of the stand.
"Regenerating aspen
Aspen tree saplings in foreground, older trees in back of photo
Aspen stands managed for pulpwood should generally be harvested at ages 45 to 55. If you grow stands primarily for sawtimber, they should generally be harvested between ages 55 and 65. You may need to harvest stands earlier if disease incidence is more than 30 percent."
Info from the U of M on MN Forests.