The term derives from the greek word 'Nymph' meaning beautiful woman. Basically a nympet is a lusty, gorgeous, young girl.
Nymphet can be applied to girls aging from nine to fourteen which is the age 'Lolita's main character Humbert Humbert lusted after.
"A nymphet crossed me in the park yesterday."
"That young girl right there is a first class nymphet!"