Techincally all of the single track trails on Stage Game Lands are illegal to ride. The reality is two-fold:
First, it wasn't the Game Commision that laid down the law - it was the Federal Govt who had a tiss about it a number of years ago with regards to funding and how said funding is used. By allowing mtn bikes the land technically becomes multi-use, which is not how the money was allocated. It's more complicated, but that's the Clif Notes version.
Second, everyone knows damn well how important mtn bikers and tourists are to the local economy and it doesn't appear anyone wants to rock the boat too much. I've yet to hear of anyone getting hassled for riding there and I have a friend who lives nearby and rides there a lot.
That said, during hunting season, it's probably best to limit riding to Sundays (no hunting in PA on Sunday).