Stonehenge is around 3-2000 BC, the iron age in, say, France started at about 700 BC, pig domestication started from beyond 10000 BC according to Wikipedia, so

Stonehenge is around 3-2000 BC, the iron age in, say, France started at about 700 BC, pig domestication started from beyond 10000 BC according to Wikipedia, so an iron age farm is conceivable. Partly domesticated pigs could also simply stay close to human settlements, without any tech necessary.