U dont even know how Madara, Izuna and Itachi mastered their Sharingan.No duh, he was older than Itachi, Itachi isn't a factor in this part of the storyline.. Obito's seven months older than Kakashi.
It was one year, because the Nine-Tailed Fox's attack happened CURRENTLY 16 years ago. And at that time, both Kakashi and Obito would be 14. There's no page, it's simple calculation of the time of events and their age.
And yes, Kakashi was already a Jonin around the time Obito "died".
Obito wouldn't be able to read the tablet, his Sharingan wouldn't be advanced enough to decipher much of anything. Oh, and let's not forget he activated it the day he died... Even the most OP Uchiha: Sasuke, Madara, & Izuna didn't master theirs' to that degree in that short period of time, that'd just be asinine. And they were the Geniuses of the Uchiha including Itachi.. And what was Obito? A below average fodder of a failure Shinobi. There's no reasonable explanation for such a drastic change in power, height, and change in voice in less than a year..
Only excuse that could be given that's also asinine is that Obito mastered the first stage of Sharingan (Let's not forget he activated the Sharingan for the FIRST time the day he died), then totally obtained the Mangekyou Sharingan and Kamui'ed himself to safety... Which is also total bs.
Maybe it was just one day and they all had 3 T, maybe it was years, maybe they were born with it o_o. But one is sure. We dont know.