Hydro, your initial thought was correct. It says "To board an attraction, children
under age 7 must be accompanied by a person age 14 years or older.
7 is the magic age for SR or just plain old riding alone.
We didn't start DS that young at Disney, but at 6.5 years old, on our last of 14 rides on Flight of the Hippogriff at Universal Orlando I noticed that they have a *height* requirement for riding alone. DS met it for that ride, and 14 times on H'griff was too many for me! So for his 15 times on Woody Woodpecker coaster on that same day he went gloriously alone. Even through the line. It was awesome.
That was the start, and at Disney the first thing he went on totally alone was the Gadget coaster. we could see the whole line from the outside, so we casually watched him from afar, saw how he behaved. He did well, had fun, so it has expanded since then. When RSR opened we did SR for it from the beginning. (so he would have been 8, wow) Especially on RSR, riding with strangers in The Most On-Camera Place On Earth (apart from Europe LOL) is not something that concerns us. We took his maturity into account more than anything; could he follow CM demands in case of a serious ride stop? Would he be at the top of CA Screamin', crying his eyes out alone, when he wouldn't be doing that if we were with him? Could he be patient and calm during a Roger Rabbit breakdown, while waiting for the CMs to come release him (we were JUST about to exit the ride to the offloading area when we had such a breakdown on that ride, and we were the second to LAST to be released from the ride...it was maddening), or would he freak out? That's the thought process we continue to go through. He still doesn't have free reign, but we used SR on Matterhorn on our recent trip and that worked well (especially since we ended up on bobsled "trains" that left one after the other).
But anyway, kids UNDER 7 have to be with someone 14+ to ride, but AT 7 they get to be alone for the attraction if they are mature and brave enough and if their parents are brave enough as well.