This theory is, at this point, (a lot) more speculation than fact, but I think it warrants being here. It's been said in the manga that the Sage of the Six Paths was responsible for the advent of ninjutsu, but he was also fearsomely powerful. The prevailing thought would be that he developed this power on his own, all by himself. Doesn't that seem strange? Surely this can't be the case, in a manga where everyone who grows stronger does so with the help of rivals, companions and mentors. This is a constant throughout the whole manga. Young ninjas are brought up by their senseis (the chuunin exam nine, for example), the bonds of rivalry are born through battle (Hashirama/Madara), brotherhood (Itachi/Sasuke), and friendship (Naruto/Sasuke), and these are just a few examples. Even the evil characters of this manga do not grow stronger alone (Orochimaru/Kabuto, Zabuza/Haku). The Sage of the Six Paths must have had people around him, strong people, who helped him along the way to developing his power.

Who, then, were the ones that shared the life of the Sage?

Possibilities include:

The ancient animals sages hidden away in their sanctuaries: we've seen the Frog Sage and the Snake Sage so far. They are a possibility simply because they are extremely old, and have enormous power and knowledge. Even if they (or their ancestors) did not teach the Sage, they may have at least shared their time with him.

One of the Sage's companions may have been a Hyuuga, whose eyes are said (by Kakashi, who may not have known anything) to be older than the Sharingan, and the Hyuuga's special chakra techniques and martial arts are not exactly ninjutsu. It's important to acknowledge that ninja techniques weren't necessarily the only way to show power.

The Sage had a wife/girlfriend, with whom he had little Senju and Uchiha. She may have been a powerful companion of his past who either helped the Sage with his techniques or was a close student.

Other ninjas, who learned of the Sage's new religion of chakra and developed their own ninjutsu, to the point where they were strong enough to call the Sage a rival. After all, Tobi made it clear that the Senju and Uchiha clans were the two strongest out of hundreds of other clans.

Family, like brothers, sisters, and parents whom the Sage could have called rivals. Maybe the Sage was the little runt of the family who swore he'd show those bullies what's what.

I should note that the Sage had to have grown extremely powerful well before facing the Juubi, and sealing the Juubi was the supreme icing on the cake.

Any thoughts on who might have walked with the Sage of the Six Paths?