The ambient temperature has the largest effect on life. The relationship between life and temperature follows a chemical reaction formula called Arrhenius' Law of Chemical Activity. The law put simply says that life of a capacitor doubles for every 10 degree Celsius decrease in temperature.
Voltage derating also increases the life of a capacitor but to a far lesser extent, as compared to temperature deratings. Internal heating caused by the applied ripple current, reduces the projected life of an aluminum electrolytic capacitor.