We report the transformation from gels to viscous fluids within a time period of 10(1)-10(3) h in mixtures of precipitated silica in silicone oil. Scaling behavior in elastic and viscous moduli versus frequency, similar to that reported by Trappe and Weitz [Phys. Rev. Lett. 85, 449-452 (2000)], has been observed for suspensions of different ages, silica volume fractions, and silicone oil molecular weights; this shows self-similarity in the stress-bearing silica network in these samples. The elastic plateau modulus, extracted from the scaling of the moduli for each suspension, decays exponentially with time, but the initial plateau modulus depends only on the silica volume fraction and is independent of the properties of the base fluids. The aging time constant is larger if thinner silicone oil or a higher silica volume fraction is used. The aging rate is slower for silica in methyl-terminated silicone oil than that in hydroxyl-terminated silicone oil by about an order of magnitude. We attribute the aging effects to the adsorption of poly(dimethylsiloxane) on silica and use a model based on the "poisons" model by Cohen-Addad and de Gennes [C. R. Ocad. Sci. Paris, Serie II 319, 25-30 (1994)] and on the simulation work by Tsige and co-workers [J. Chem. Phys. 118, 5132-5142 (2003)] to explain our observations. No aging effect has been observed for silica-mineral oil mixtures. (c) 2007 The Society of Rheology.