Old. Stonehenge is old. It has something to do with Druids and astronomical calculations, and it's out in the middle of nowhere (aka Salisbury Plain.) That's about all I knew about Stonehenge until I read Mike Parker Pearson's enlightening book, Stonehenge: A New Understanding: Solving the Mysteries of the Greatest Stone Age Monument.
How old? Well, that's not as easy to answer as you might think. It's a little like asking, "How old is Troy?" It depends on which Troy you're talking about, because there were a series of Troys, exposed one by one as the archaeologists dug down into the layers of the continually rebuilt city.
Stonehenge is much the same. There were five Stonehenge stages, with different stones and ditches and ridges as Neolithic, Copper Age, and Bronze Age peoples built and rebuilt the monument. It was begun in about 3000 BC and was abandoned in approximately 1520 BC. (May I just remark here how awed I am at the scientists who have been able to determine these dates and the centuries of work that have gone into the study of Stonehenge. Inigo Jones and Flinders Petrie among others studied the site.)
What about the Druids? They did not build Stonehenge. Scientists have irrefutable proof that the Druids had nothing whatever to do with Stonehenge. This does not stop people calling themselves "Druids" from considering Stonehenge theirs and petitioning the government to rebury the bones found in the area of the monument. Nor, for a time, did it stop UK government regulators more concerned with the possibility of injuring the feelings of New Age pagans than with hard science from ruling that the bones had to go back into the ground.
When American Indians complain that scientists are removing the bones of their ancestors from sacred ground and plead to have them returned it makes some sense. These really are ancestors, those really are sacred places for their tribe, and they can prove it. That self-described 21st century "Druids" should be given the same respect is ludicrous. Fortunately the government reversed their position and allowed archaeologists to proceed to conduct the 2003-2009 Riverside Project, which has vastly increased our knowledge of Stonehenge.
Is Stonehenge in the middle of nowhere? Well, when you look around when standing at Stonehenge you don't see a lot except for a highway that was build very close to the monument. (Who was the bureaucrat who allowed that to happen?) But as Pearson's book shows, Stonehenge was build in a complex of henges, some wooden and others of stone. A reconstructed map shows well over a hundred Stone Age henges, wooden circles, palisades, stones, barrows, trenches, burials, houses, and walls (indeed an entire village) in the area called the Stonehenge-Durrington Walls complex.
Astronomy? Indeed, Stonehenge stones line up perfectly to catch the midwinter sunset and the midsummer sunrise. The astronomer-priests who built it knew what they were doing. And now, thanks for people like Mike Parker Pearson and the other scientists who worked on this project, we know what they were doing too.