I just took a pleasant, but cautious ride at the Underdown. Bike trails are open basicaly until there is enough snow to ski and snowshoe. Right now there is no mud to worry about, but there are a few trees that have blown down.
The only time the trails are closed is during April, though I wouldn't ride in daylight during deer season. Trails open for the spring on May 1st. By then most of the leaves and logs should be off the trail.
I'll probably sign the snowshoe trails on December 23rd. In the mean time I'll be getting ready for Tuscobia.