This study focuses on the influence of sludge age (SA) on the production of electricity from a cyclically fed glycerol–based microbial fuel cell. Under the same hydraulic retention time, different volumes of sludge were extracted from the anode compartment, thereby modifying the SA. Such changes affect the electrochemical performance, the organic matter biodegradation and consequently, the coulombic efficiency. A sludge volume of 0.01 L (corresponding to a SA of 24 d) appears to be optimal, because this favors the development of electricity–generating microorganisms (EGM). Shorter SA times wash EGM out of the system and promote growth of the fermenter (mainly acidogenic bacteria), whereas a longer SA reduces the microbial population. A final product analysis identified that short SAs provide favorable conditions in which higher concentrations of short–chain organic acids are detected.