The age of permafrost is closely linked to the time required for soil systems to freeze, since the permafrost must be at least as old as the formation time. Cycles of freeze–thaw will complicate the relation between the freeze rate and the age. A model based on pure conduction heat transfer with freeze–thaw is used to predict the time required for a given thickness of permafrost to develop, either heterogenetically or syngenetically. The formation time is a function of the long-term geothermal gradient (initial temperature of the thawed soil), the ratios of the frozen to thawed thermal properties, and the temperature history of the upper surface of the permafrost (higher than the air temperature). The simple theory allows universal graphs to be produced that predict the formation time for a given thickness of permafrost. Realistic soil property ratios and paleotemperature scenarios will then lead to estimates of the formation time of permafrost for a specific site. The model indicatesthat deep permafrost (more than 1500 m) requires formation times on the order of the complete Quaternary Period.