Mary found a small vial that read "Age Regression." On it. Beneath there was instructions and little markings indicating how many years each sip of the vial would take away.
"Hmm, I guess I'll try this one," S he thought. "Just a small sip, see what being 21 looks like." As she tilted the vial back however, a little of the fluid dribbled to her hands and they slipped, making Mary gulp down more than she had wanted to. She gagged, coughing and glared at the liquid. It tasted awful and she wiped her face. Nothing happened.
"Hmph, should have figured these were fake," She thought looking at the line it stopped on. "Woulda sucked to be 12 again, sheesh."
Suddenly, Mary felt a lurch in her body, and her stomach began to churn. She felt cold all over, even as her body broke out into a sweat. Before she could react, she fell down to the ground, coughing, her whole body shivering as she lay upon her own carpet.
Mary's mind was abuzz with frantic questions, wondering if she was sick or poisoned, but even as she wondered these things, she saw her hands. Rather, she saw her hands starting to shrink. She watched spellbound as they became smaller, daintier, and she felt her shirt starting to become larger on her body. Rolling onto her side, Mary managed to crawl towards a mirror and stopped short. The face reflected in the mirror was her in her early twenties, the lines of age swiftly being reduced. Her hair was regaining natural highlights as her breasts were becoming more perky.
"It's working, I'm becoming young again." She thought excited. But then the reality hit her. "But that means..."
Almost as if on cue she felt a wrenching in her chest and watched helpless as her breasts began to grow smaller. She felt her hips starting to push inwards as her body became smaller and more lythe passing through puberty. Inside she felt her guts churning as she regressed back through high school, pimples dotting her face as she cried out in agony, voice raising an octave as she reached about fifteen.
She fell forward a hand on the mirror, and stared at her face. Slowly, the pimples dissapeared one by one, and her face began to soften back into that of her preteen self. Freckles began to appear around her nose as it turned up, and all signs of her breasts vanished. Her clothes hung off a gangly, coltish body now as her hair fell short to her neck. The changes done, the sensations ended, and Mary was left in utter shock.
For all purposes, she looked like a 12 year old trying on her older sister's things. Unable to cope, she blacked out and fell unconscious.