This paper provides some insights on aging due to PD activity occurring in insulation cavities. In particular, it is observed that the variation of PD amplitude may drastically decrease or increase during aging, thus challenging the interpretation of PD magnitude and repetition rate as diagnostic quantities. Explanations are proposed, based on a series of chemical-physical measurements, as Raman spectroscopy and SEM observations. At the beginning, aging leads to the formation of crystals on the cavity walls, which decrease PD activity, while for longer aging times also pitting and electrical treeing is observed. This would lead to speculate about the diagnostic validity of spot off-line measurements.