Volcanic plains are the most common surface units on Venus. A survey of distinctive contrasts of the radiophysical properties associated with boundaries of volcanic units was carried out. Only those sites where the sign of the dielectric permittivity difference can be reliably inferred from radiophysical data were considered. The inferred contrasts in the dielectric permittivity across the unit boundaries were correlated with the relative stratigraphic positions of the units. It turned out that locally older units usually have locally lower dielectric permittivity. This phenomenon can be a result of a set of processes of surface alteration and mantling formation, which decrease the dielectric permittivity with time. The observations are not consistent, however, with a gradual and globally uniform decrease of the permittivity. Thus it is impossible to instantly use the radiophysical properties to date the surface material. The results show that further studies of the radiophysical data sets from the Magellan data collection are promising for understanding the surface mantles and mechanisms of their formation and evolution as well as for some inferences about ages.