As Nicky kept looking down at his much older friend, he began to rinse out the last of the shampoo from his head, solidifying the changes Marcus went through. The gruff fat man, sat on his fat butt on the ground. Clearly exhausted from the energy he used on Nicky.
Nicky spent some time looking over Marcus, his changed body, how his best friend was now a very old man. A very attractive old man. Not wanting to leave his friend alone in the life he would now lead, he grabbed the shampoo bottle, and lathered some onto his hair.
The tingling sensation from when he first used the shampoo began to spread across his body like a forest fire. He ft his body began to expand outwards more with muscle as he slowly crawled into his mid 20s. Going from a nice toned figure to looking more and more like a amateur bodybuilder with a clean cut body. His beard grew in thicker with his body hair creeping around forming a distinct treasure trail and chest fur.
Going into his thirties his frame solidified and then slowly deflateing as he entered into his forties, as his body quickly began to form a dense layer of fat all over. Aging into his 50s his hair began to lose a small amount of it's deep red coloration to a speckling of grey, and hair began to appear in various places like his nose, ears, armpits, and back. All the while he grew softer and softer as his muscular twenties gave way completely to an obese 50s. He probably matched Marcus now in weight with a considerable belly with rolls and moobs.
Passing into his sixties was where he decided to wash out the shampoo. Wiping the last out of it left him an old fat man like his friend. He didn't go full grey, and he was lucky enough to still have hair on his head albeit noticably thinner then his youth. The transformation alone took most of what energy Nicky had left. He lowered himself to the ground, and sat up against Marcus and fell asleep resting his head on his old friends shoulder.