Did not ALL our mefarshim have RH”K? Most do not agree with each other, especially Ramban and Rashi. Their perushim are very important and give us a tremendous understanding of what the Torah probably means by a given expression, but not everything can be correct, especially when two or three opinions are completely opposite. They are opinions, not halacha. It does not matter how old Rivka was, waht matters is that she was an amazing person, even as a young child, filled with chessed and sensitivity, and demonstrated the right middos that made her deserving to be one of the Emahos.
