I have a Grandfather that was Endowed at the age of 15 in 1879 in the St. George Temple. I cannot find out a reason from FamilySearch as to why he would have been that young. Is there any reason, from a Church History perspective, for this?

I'm no historian, but it wasn't unheard of for people to be endowed quite young - especially if they lived a distance from the temple. I have several ancestors who were endowed at 15, 16, 17, they were also sealed to their parents and married(!) around the same time. In 1879 St. George was the only temple and travel was difficult. Unless he already lived nearby, it's possible he was endowed when he was there because it was unknown when/if the opportunity would come up again. Remember there were no recommended ages for such things at the time, and boys finished school and started working at 14. If the bishop felt he was mature enough to understand and be faithful to his covenants, he could be endowed.

The age for receiving those ordinances were not as strict at that time. Also he may have been going on a mission, or he may have been getting ready for marriage. They sometimes did those things at a younger age. He might have also been exceptionally mature.