Quality of construction and personal care go a long way to maintain the life of an AEG, but one golden rule is obeyed by the mechanical nature of our hobby:
The more you cycle the mechanism, the more energy it absorbs in impact, the shorter your AEGs life becomes. Mechanical life of an AEG is not measured so much in time, but in how many shot cycles you can put it through in a vacuum.
You should be fine with a 9.6v battery but just realise the above statement is how the mechanical world works in general.