'The Broch Age' was roughly 500 B.C.E. to 500 C.E., but there were some around 700 B.C.E. and perhaps even as far back as 900. Of course quite a few could be built over even earlier structures, but archaeologists have a habit of only going as far back as the first monumental building and the ceasing excavating in order to preserve this ! This has been well argued by Dave Lynn in a paper yet to be published, along with a few examples of brochs known not to have been built on 'greenfield' sites e.g. The Howe