Nina was unpacking the groceries, setting aside a few items on the edge of the counter. Nicky her son opened the back door, his clothes covered in mud, dirt smeared over his face. Nicky was ten, and had his mother's red hair, but it was dirty enough to look brown at the moment. "You better not be thinking of tracking that mud in my nice clean house, young man." Nicky stopped as soon as he heard his mother's voice, yanking off his shoes before stepping inside. Nina looked up at her son and shook her head. "Out in the woods again?" She didn't even wait for her son's reply before going on in a brisk voice. "Your father will be home soon and his boss is coming over for dinner, upstairs and take a bath, now." As Nicky moved to obey she halted him again. "Wait." Picking up the bottle of blue shampoo she tossed it to him. "Now get."
Nicky tromped upstairs, his muddy shoes in one hand and the bottle of shampoo in the other, muttering to himself about not being that dirty until he heard his mother's voice calling from downstairs in that tone that expected his to behave. He couldn't even believe she had heard his griping. He slipped into his bathroom, his older brother had left for college earlier this year, and he was enjoying having the whole bathroom to himself. He peeled off his muddy clothes and kicked them in the corner, he'd probably hear about it later, but he didn't care right now. Most of the mud was on his clothes, only his arms and face were really dirty. He turned on the shower, feeling it until it was just right before finally jumping in. He let the water wash off a bit of the dirt before grabbing for the shampoo and squirting way too much into his hand. He dug his hands into his hair, keeping his eyes closed as he lathered up.
He couldn't see what the shampoo was doing to him as he worked it into his hair. It tingled more than usual, but that was nothing to startle him. The shampoo did as it was supposed to, well, as the doctor had designed it. It began to age him, and right on the edge of puberty like he was, the effects were instantly noticeable. His small childish penis was the first thing to show any sign. It grew a bit, especially his balls, reddish hair growing in just around it. His legs and arms stretched out, giving him the look of a boy growing taller quicker than his body can keep up. Hair began to darken around his ankles and wrists, his hands and feet also getting awkwardly bigger. A little grew under his arms as he put on a bit of muscle, still looking horribly gangly though. He stuck his head under the rushing water letting the shampoo rinse out.
He felt his hair, the shampoo hadn't really cleaned it well. He must have been dirtier than he thought. He squirted some more in his hand, working it once more into his hair, eyes still shut tight to keep the shampoo out. He decided to leave it in a bit, just letting the water wash over him. His penis continued to grow, faster now than before, inch after inch it lengthened even as his balls grew in size, only slowing once it neared six inches. His pubes thickened even further, a bush on his crotch rather than the few straggly hairs that had been there before, it even peaked, making the start of a trail to his belly button. The height growth slowed, only to be replaced with sideways as he filled out. His shoulders got broader as lean muscle packed on his growing frame. The hair on his legs and arms continued to darken, moving up his limbs slowly. His face began to change, the child-like features giving way to something more masculine, but still the appearance of youth remained on him. A few hairs grew in on his chin and upper lip as well as his sideburns. And a slight scratch in his throat he coughed to get rid of signaled his voice changing.
He washed the shampoo out of his hair again, blinking his eyes open and gave a start. His face was right under the shower head, he wasn't that tall. He looked down and saw himself, his eyes going wider. "Holy crap..." Which surprised him even further, his voice was so deep. He jumped out of the shower, leaving the water running as he dashed to the mirror. He touched his face as he examined himself, still unbelieving. He looked fifteen or sixteen, or at least close to how his brother looked at that age, and damn handsome if he said so himself. He grabbed the shampoo bottle and looked it over, it seemed like the same shampoo he always used, but this had to be what did this to him. He looked in the mirror again, seeing his own surprised face slowly break into a grin.