According to Live Science, trees in the forest are at least 1,000 years old. Stumps and logs coming from the thawing glacier were first noticed 50 years ago...Thanks Islander7:
The researcher feels comfortable putting the forest at 1,000 years old, and some trees have tested even older. The oldest tree Connor has found on the Mendenhall Glacier is around 2,350 years old.IOW, 1000 years ago the climate there was warm enough for live trees to grow, without glacial cover. This evidence (like all the other evidence) confirms that the climate is 100 percent natural, 100 percent of the time.
If it wasn’t for global warming, Chicago would still be under a mile of ice.
My guess is they are a little less than 1000 years old, which would put the forest squarely in the Medieval Warm Period.