In this paper, we report our results on the electrical properties and aging phenomenon (or resistance drifts) of Negative Temperature Coefficient thermistors manufactured out of manganite ceramics of general spinel structure: Mn3 - x -x'MxNx'O4 (0 less than or equal to x + x' less than or equal to 1; M and N being Ni, Co or Cu). It is shown that the resistance drift is not a result of the use of metallic electrodes or metal/ceramic interfaces. However, thermal treatment used to bond metallic electrodes on ceramics triggers the electrical aging. Components with almost no electrical drift can be obtained by carefully controlling this metallization treatment. Beyond this experimental result we have tried to determine a suitable basic origin explaining the aging of NTC. Depending on the studied solid solutions, i.e. cations involved in the spinel structure, many overlapping complex ionic diffusion mechanisms might be operating. However, our study suggests that electrical aging might be triggered by the high mobility of Mn3+ cations which have the tendency to gather in clusters in such oxide structure.