From The Force Awakens we know that the following events took place at some time between RTOJ and TFA.
1) Rey is abandoned on Jakku by someone.
2) Ben Solo is seduced to the dark side by Snoke, joins/creates the Knights of Ren and betrays Luke Skywalker, the man who has trained him up to this point.
I guess there is a good chance that the answer will be obvious come Decemter 15th, but I wondered whether there is - as of now - enough canon information to figure out which of those two events happened earlier. I have gathered so far:
In her flashback, Rey does not look older than ten years old, when she is abandoned. Actually she does not look older than the young Jyn Erso in Roque One, who was known to be eight years old at that time. Since Rey is born 15 ABY, this would mean that event 1) did not happen after 25 ABY. This is of course only a reasonable assumption and not a confirmed fact.
During the events of the canon novel 'Bloodline', which is set to take place 28 ABY, Leia is at least not aware of the fact that Ben would have already turned to the dark side. Furthermore it is stated that Ben does not even know that he is the grandchild of Darth Vader up to that point. But Leia never talks to him during that novel, so I would not take it for granted, that Ben is still Ben at that point.
However, assuming that Leia would immediately feel Ben betraying Luke and killing his trainees through her connection to the force (like she immediately felt him killing Han), one would conclude that Ben's betrayal did not happen before 28 ABY and consequently after Rey has been abanoned.
Is there currently any more canon information to back this up?