If the wind is straight down the runway and not gusty, even a beginner should be able to take off, fly, and land without trouble so long as the wind speed isn't higher than the plane's normal landing speed. The wind even makes landing easier, because your ground speed on final is low. Add a crosswind, and some experience becomes important. If the wind speed and direction are constantly changing, it stops being lots of fun at maybe 15 mph. Another factor is the environment at your field. I used to fly from a field that had a tree line running about half the length of the runway and parallel to it, on the north side. It was exciting when you had a north wind, because the trees would block it at low altitudes, but only to about the middle of the runway. So there's no simple answer. For beginners, I usually suggest about 12 mph, but as a guideline, not a fixed rule.

Air Sports Weather (www.usairnet.com) is a good forecaster of wind speeds, but they don't do gusts.