The word देवी comes from the root दिव् which means they are made of light.Alternatively, they are the ones who advise pravitti and nivritti to all.
देवी¦ स्त्री देवयति प्रवृत्तिनिवृत्त्युपदेशेन यथाधिकारं व्यव-हारयति सर्वान् देव--णिच् अच् ङीप्, दीव्यति दिव-अच् पचादौ देवट् इति निर्देशात् टित्त्वात् ङीप्[Page3750-a+ 38] देवस्य पत्नी ङीष वा। Ref :https://sa.wiktionary.org/wiki/%E0%A4%A6%E0%A5%87%E0%A4%B5%E0%A5%80
They take forms only when required. Natures are mentioned in the dhyanamantras. For example Devi Vishalakshi is known to be very angry.The age is also mentioned in some dhyanamantras.
The dhyanamantras of many devis are available in Vrihattantrasaara,Niruttara-Tantra, Kalika-Puraana etc.For examle, Vishalakshi is mentioned to be always sixteen years old. Devis are not bound by time and so they do not age.Dhumavati is depicted old. Otherwise normally all devis are purnayauvana. Their complexions are also mentioned in the dhyanamantras.
Reference : Bharatiya Shaktisadhanaa o shaaktasahiya, Upendrakumar Das, Vol.1, RMIC, page 534-537).