Shisui was still alive when Tobi attacked Konoha. At that time Tobi already had Obito's eye and Shisui still had both of his. So enough of these stupid Tobi=Shisui theories.
It's not going to be Shisui for the simple fact that it contradicts him loving Konoha so much. The guy tried to genjutsu the Uchiha clan to not attack Konoha. Now does it make much sense for him to go through all that effort only to have his eye stolen by Danzo afterwards and then try to destroy the village years later when that was the very thing he was trying to prevent? He also wouldn't just hand over his other remaining eye knowing how useful it was. Tobi is the type to steal powerful eyes, not just give them away like a stick of bubblegum.
Right now Kishi is trying to set up the fans to believe he's really Obito but I'm not 100% convinced. He's done the same with Madara. There's a good chance he may not be an Uchiha even now. Why? Because ok sure he has an MS, but his eyes never deactivate just like Kakashi who is an expert with his sharingan as well. Nagato kept his rinnegan on all the time with no sweat despite not being an Uchiha. We don't know if Tobi is human since his arms tend to melt like icecream but we do know he has Uchiha and Senju dna. It wouldn't suprise me if he even used Uzumaki dna to increase his chakra stamina.
It's not possible simply... Don't even wanna prove it, you will see it soon enough
And it has nothing to do with his age, and that + hair ressemblance being only prove is just sad. Glad all these theories can soon come to an end, they are getting worse and worse.
It's not Shisui. Remember when tobi confronted Danzo and he said "Shisui's eye is mine" or something related to that. If it was Shisui why wouldn't he say "my eye". I highly doubt this theory.