Yes, the Ranma as a prodigy genius who skipped grades and learns martial arts moves super fast theory actually works out quite well for several reasons. First, it explains the common reaction of jealousy towards Ranma, after all, who can understand someone two or more years younger than them being stronger and smarter than them.
It helps explain some of Ranma's debatable ignorance a bit better.
Timeline wise, Ukyo seemed older and more mature in the flashbacks with the two of them, Ranma left Ryoga's Jr. High school for China, and in the flashbacks seems to be about 12 or 13, and it seems unlikely they were in China for very long, otherwise they'd have learned Chinese, or at least more Chinese, even if it's just through exposure. Kasumi's comments about Ranma's youth, Ranma's often childish attitude is better explained if he's a couple years yonger than Akane, how he keeps up in school at all when he's so lazy about schoolwork and has such a chaotic lifestyle, and so on.
Just because he's a genius doesn't mean he has good social skills and is well educated though, because of his focus on martial arts and skills appropriate to a traveling martial artist.
Thus, I agree with your theory that Ranma is about 14 years old. He could have easily gotten into Furinkan with how low it's standards seem to be, and the fathers would have wanted him in Akane's class rather than a Jr. High Student if possible. As far as I know, Ranma's age in the series is never explicitly stated. It would be humorous for someone to point out in a fanfiction taking place after the failed wedding that Ranma isn't old enough to get married legally anyway!
