This movie was made in the 70’s or 80’s. It was about a Doctor/Scientist who was trying to genetically engineer a superior human. He succeeds and the girl develops rapidly. As she reaches adulthood, they begin an affair. She starts to exhibit troubling behavior and appears to be aging rapidly. She becomes more unstable and he ends up (I believe) killing her.
Sounds a bit like Embryo (1976).
The poster above is right. This definitely sounds like Embryo.