Peggy is still played by an adult Kathleen Turner once she Mental Time Travels to her high school years because that's how she still sees herself
Once she goes back in time, the rest of the characters see her as looking exactly like an actual teenager, and the fact that the audience doesn't is related to the movie being from her POV: Either she would literally see her adult self in the clothing and hairstyle she wore as a teenager if she saw her reflection, or else it just symbolizes that, due to still having her adult memories, she feels like an adult trying to masquerade as a teen.
