Technologies envisaged for a 5G communications system provide interesting prospects which are beneficial for co-operative positioning. Since 5G development is in the early stages, there is a unique opportunity to develop and integrate mobile radio based positioning technology in 5G from the beginning. In this paper we discuss 5G concepts to ensure seamless positioning by cooperative positioning. In cooperative positioning, mobile terminals (MTs) collaborate to help each other to determine their own position. We address key 5G prospects like smaller cells, higher MT densities and the capability of device-to-device (D2D) communication to enable cooperative positioning. By using the Cramér-Rao lower bound we investigate cooperative positioning performance for signal propagation delay based pseudo ranging in an exemplary typical urban environment. Numerical results for an exemplary environment have shown that with MT densities D > 1100 MTs per square kilometer sub-meter positioning accuracy with outage probabilities converging to zero can be achieved.