Yeah Obito being a teenager is a problem. It doesn't make sense he would obtain a jump in abilities from being a nobody in to ninja to rival a Hokage. Even Sasuke, a genius from the start, has taken time to get to where he is without having one of his eyes gone and part of his body crushed.

Kurama spoke about Sasuke's chakra being more sinister than his own, like Madara's was. Yet in the case of Tobi he doesn't imply Tobi is like Madara but that he IS Madara.

Izuna wearing a helmet? Well to hide his identity for some stupid Uchiha clan related reason. Not to mess up the legend of the two powerful brothers. How would the political aims of Madara be served (to get the Uchiha to revolt) if they knew his brother in cahoots with him was going arond stealing eyes?

Apart from all this the reason I'm not conviced it is Obito is because it sounds too straightforward. I know people for ages speculated about Minato being Naruto's dad and it turned out to be true, but we are approaching the end of the manga now and it would be more dramatic a twist for everyone to be led down one path and then being wrong footed to the very end. I'm not completely ruling out Obito, as we are all just speculating, but I think it would serve the series better for it to be Izuna. Plus that may explain why Madara is aware of Tobi. Additionally, it would serve a greater purpose for Madara to remain present in the story fighting the Kages than to just be the big bad villain no one can seemingly beat.