[[ The Original Shin was an experiment through the combined efforts of Danzo and Orochimaru to create a human whose DNA would allow Sharingan to be easily transplanted in any part of the body. Shin apparently heard a LOT of praise towards Itachi from the both of them as he fully believed that Itachi was the epitome of everything an Uchiha should be. 

Shin lost his right arm which in turn became Danzo’s right arm.

At some point Shin escaped Orochimaru’s facilities. The most believable time frame for this is when Sasuke killed Orochimaru, leaving most of his bases completely unattended. 

Shin later discovered that Sasuke had killed Itachi. Because Shin lacks the information that Itachi was a wanted criminal for the massacre of the Uchiha and that Sasuke was the only survivor, Shin mistakenly believes that Sasuke murdered Itachi, and therefore his enemy became Sasuke. 

Shin began cloning himself at some point using parts of his body, but the small child-Shin turned on him during the fight versus Naruto, Sasuke, and Sakura, and they killed the adult. 

I don’t know his age– not even the wikia does. It’s probably trivial because due to the existence of Mitsuki and Log, Orochimaru is able to produce subjects that don’t need to go through infancy/childhood. ]]

[[ I already do :3c it came to me in a dream, once. ]]

[[ No, besides intel somehow making its way to him, likely via other methods, like crows, he didn’t really check up on him. I think at that point he assumed Sasuke was strong enough to handle Orochimaru and not actually end up dead. Given Orochimaru wanted him as a vessel, he’s not likely to make any attempts on his life for a few years. So there was a bit of trust in Sasuke and faith in his own plan, there. ]]