Distributed energy resources (DERs) in distribution networks have great potential for providing capacity reserves to the transmission systems. In this context, this paper focuses on optimizing the aggregated flexibility of DERs in power distribution networks, involving both network-wide voltage constraints, and individual device operating constraints. The intended task is first formulated as a convex program, and several critical properties of the underlying optimal aggregation solution are then highlighted via rigorous convex analysis. Leveraging the special problem structure, a decentralized solver is further developed to efficiently find the optimal aggregation trajectory with limited communication. Performance tests on the IEEE 37-bus benchmark are conducted to verify the theoretical findings and demonstrate the impact of various network configurations.