The Causeway Forts were connected with the Rammas Echor, the Wall of the Pelennor. It seems reasonable, therefore, to assume that the forts were built at the same time as the wall. We're not told exactly when that was, but the weight of evidence suggests the year III 2984 or shortly thereafter (see note 1 to the entry for the Rammas Echor for the reasoning behind this date).

Of course, it is not impossible that the forts already existed when the wall was built, and were simply incorporated into its structure. The Causeway from Osgiliath must have already existed when the wall was built, so it is not implausible that it had forts to guard it. If this possibility is correct, then the forts might have been built much earlier than III 2984, and perhaps even dated back to the earliest days of Gondor.