The sources of jizya and the practices varied significantly over Islamic history. Shelomo Dov Goitein states that the exemptions for the indigent, the invalids and the old were no longer observed in the milieu reflected by the Cairo Geniza and were discarded even in theory by the Shāfi'ī jurists who were influential in Egypt at the time. According to Kristen A. Stilt, historical sources indicate that in Mamluk Egypt, poverty did "not necessarily excuse" the dhimmi from paying the tax, and boys as young as nine years old could be considered adults for tax purposes, making the tax particularly burdensome for large, poor families. Ashtor and Bornstein-Makovetsky infer from Geniza documents that jizya was also collected in Egypt from the age of nine in the 11th century.