Explanation on the R=U theory?

[Jimmy Dark Aeons Slayer]

Ok but if Rinoa was in fact Ultimecia and assuming she survives until Ultimecia time, then shouldn´t Ultimecia be an old woman?
In the present time Rinoa is 17 so in the future (and assuming the 50 year difference mark) she would be 67. My point is that Ultimecia does not look in any way like a 67 year old woman she looks like she is her late 20s or early 30s,so unless Rinoa was frozen in some place there is no way she could be Ultimecia.

Actually she'd have to be a lot older than that I think. I'm not sure how specific the dialogue is, but I always took it that Ultimecia existed at least 100 years after the game's main events, I can't actually remember why though.
It was generally taken (at least by me ), that Sorceresses either had longer lifespans than normal, or could sustain themselves magically. Evidence for this being the fact that Edea doesn't look nearly as old as she should, and arguably neither does Adel. Ultimecia in particular has a special relationship with time, though I can't remember exactly what it was, so I can't really back much up with that. I think the story of Hyne also might point towards such survival capabilities, but again, I've forgotten it.
Anyway, I don't know exactly what Ultimania says, but it seems to discredit at least a lot of the above speculation.

Well you have to consider that Edea is in her late 30s and as such her image corresponds to a late 30s year woman, as for Adel she was indeed in a "frozen like" state which would explain why she remains the same.
And the explanation that you give about Ultimecia is correct to a certain extent, Ultimecia can launch her "spirit" to the past but she cannot send her body and as such she would age if she was Rinoa and could not remain that young.