In the differential diagnosis of Spitz nevus vs malignant melanoma, patient age provides a critical piece of clinical information, because Spitz nevi occur mostly in children and melanomas occur mostly in adults. Nevertheless, there is overlap in the age distributions of Spitz nevus and melanoma. The issue to consider is how these age distributions and their governing probability densities can impact the a priori probability that a lesion is a Spitz nevus vs a melanoma. Herein I introduce a quantitative approach that uses Bayes rule together with previous published data on the age distributions in Spitz nevi and melanoma. The resulting algorithm yields plots and a table of predictive a priori probabilities of Spitz nevus, given patient age occurring within narrow intervals, and I believe these provide useful guidelines for using age in the differential diagnosis of Spitz nevus and malignant melanoma.