People keep complaining that tobi can't be obito because he was too young to give Nagato the rinnegan. Really what are you talking about? Obito/tobi told konan that the eyes where his to begin with but what you are forgetting is he was still playing the I am uchiha madara card. Nobody knew he wasn't madara til the real madara was edo back except for kabuto.
And for the complaint he was too young to fight the forth it probably wasn't him it was madara. Nobody in naruto changes their hair that drastically. That first masked man had madara hair. Madara prolly wore the mask because he knew he was close to death so it would be easy for obito to assume his role so easily. Obito had to survive the cave in when he was young and prolly went to find kak and Rin only to see Rin had gotten killed and kak getting praised for being the hero with the sharingan. He was filled with rage that kakashi didn't protect Rin therefore hating the current ninja world and wanting to do away with war lie and pain by any means.
He some how meets madara and begins to train under him to improve his skills. Then madara tells him the plan to revive him when he dies. Obito had to have met madara he wouldn't choose him out of all people to impersonate if he never even met him.