Selective Mutism occurs between ages 3-6 (see the concept node "Incidence and Prevalence of Selective Mutism"), whereas Social Phobia typically begins between ages 11-13.

A particular level of cognitive development is required for the development of Social Phobia.

Selective Mutism's earlier age development is inconsistent with the development necessary to be considered as Social Phobia.

Melfson notes that the typical SM age of onset coincides more closely with the development of shyness (which typically manifests at age 4), a weaker subcategory of social phobia.