A composite-forming process includes impregnating a reinforcing structure with a curable composition at a temperature of about 10 to about 40° C. The curable composition includes specific amounts of an epoxy resin, a poly(arylene ether), a solvent, and a curing promoter. The poly(arylene ether) includes, on average, about 1.6 to about 2.4 phenolic hydroxy groups per molecule, and it has a polydispersity index less than or equal to 2.2 and an intrinsic viscosity of about 0.03 to about 0.2 deciliter per gram. These characteristics substantially improve the solubility of the poly(arylene ether) in the curable composition and allow the curable composition to be formed and used at or near room temperature. Composites formed by the process and circuit boards including the composites are also described.