Android 18 was 20 years old when she and her twin brother (android 17) were enlisted on teh street .#18 and #17 forgot all memories from there original life, like names, original location, parents etc. And Dr. Gero programmed them to become the "perfect" androids. They were quite powerful due to the fact 17 could easily knock out Trunks, and 18 could defeat Super Saiyan Vegeta! But 18 forgot her purpose to destroy Goku and the Z fighters when she fell in love with a short, bald-headed monk, his name was Krillin. The human part of her body took over at one point and she gave Krillin a peck on the cheek, and then the Android part took over again. But later the human part almost completely takes over when Krillin saves her life, when Cell regurgatates her, and then he uses a valuable wish to get rid of her self-destruct device. After that we never really know what happens, except they get married and have a baby girl named Marron. Later, Android 17 comes back from hell as Super Hell Fighter 17 and kills Krillin, this enrages #18 and she fires a volly of energy blasts that android 17 absorbs, to 18's surprise. She receives Goku's help as she continues with the volleys of energy which destract him, as Goku punches threw his chest, killing him. 18 then wishes Krillin back, but later they both die of old age, and they ask not to be wished back, so they stay in the Great Snake Way forever ::sniff::
Android 18 doesn't visably age throughout the entire series, she never looks old, she just gets old.
Android 18 is 3/4 human, unlike her brother who is 3/4 android.
Android 18 never takes off her pearls throughout the entire series!
Android 18 was the least powerful out of 17, 18, and 19
Android 18 never shows her affection for anyone, until Krillin dies. Even though she felt it for both him, Marron, and the girls of DBZ, she even admits that Bulma, Chi Chi and Videl are her best friends later on.