The sage of Six Paths was the creator of jutsu ,but hasn't anybody wandered if it's possible that he was able to use all the jutus we have seen so far in naruto including his rinnegan abilities because being that he's supposed to be the creator of jutsus it would make sense if the sage was able to use every jutsu in the naruto world.
If the SO6P had the Rinnegan, and if the Rinnegan is the last stage of development for the Sharingan, that means the Sage had the sharingan when he was younger. If the rules were the same, the Sage would have needed to awaken his sharingan through battle stress, awaken the MS by killing his best friend, getting his EMS by stealing an MS from a family member, and then awakening his Rinnegan through unknown means.
He was probably a martial artist of some sort. People would have been aware of chakra, but wouldn't have known how to use it correctly. The Sage taught them about chakra and how to use chakra to make jutsu.
With his sharingan and Rinnegan he would have been able to see chakra, including the flow of chakra and the color of chakra. He would have been able to see how hand signs manipulated the chakra, and would have been able to see the results. Some basic experimentation, with the Rinnegan on his side, would have been able to develop all the major theory of jutsu.
If he also had Senju power, he would have had plenty of power to experiment with and would have learned that training would increase his chakra levels. All he would need is a way to increase his power high enough to confront the 10-tails, then as a jinchuuriki he would have had all the chakra he would ever need. Assuming he had a long and healthy life, he would have had plenty of time to learn many secrets of jutsu.
