This is a very good question. It is probably a consideration that gave Steinbeck some problems when he was plotting the story. I believe that he took pains to make Curley's wife as young as he possibly could. In her monologue when she is with Lennie in the barn she says that she almost ran away with a traveling "actor" when she was fifteen and then that she married Curley a short time later when she met him at a dance hall. One of the farm hands refers to her as "the kid." Her dialogue shows that she is a high school dropout and is not terribly intelligent, and that she is certainly naive and inexperienced. She doesn't understand much of anything. She doesn't think. She lives in a dream world. She seems promiscuous and flirtatious only bcause she has fantasies of being a movie star and is practicing her seductiveness on men. Steinbeck wanted to create a character whose accidental death at the hands of Lennie would seem plausible. That was why he made her so young. Steinbeck doesn't specify her age, but she is probably only sixteen. It was also plausible that Curley would marry such a young girl, because Curley has a big inferiority complex and might not be able to relate to an older woman. An older woman might sense that Lennie was potentially dangerous, especially after the way he had mangled Curley's hand and the way he has just killed his own puppy. Even though Curley's wife is very young and very naive, it is still a little hard to understand why she would move closer to him in the barn and let him move closer to her--and then invite him to feel her hair.