Even though the film itself was nice and had quite of a growth for <PERSON> that one duel and also a scene before are very strange. But lets start with the beginning shall we?
He is too old you say The too old I always got the feeling that it was just an excuse so that the one who was declined does not feel that bad about it. In addition to that though the older one is the more bonds one has formed already and Jedi shouldn't have bonds as those can be used to make them fall to the dark side.
The 1 scene I mentioned
! The interrogation scene with <PERSON>. <PERSON> was able to get into her mind until he made an error and said something that made her will go strong. At this time she EASILY overpowered someone who has trained such things FOR YEARS. As we know from the prequels raw force potential says nothing (<PERSON> lost to <PERSON> in Episode 2......easily) without any training. So there are 2 options for this part. Either <PERSON> had a training and forgot about it (during her unknown childhood part before <PERSON>) or she has a WAY HIGHER force potential than <PERSON>. High enough that she is even untrained way more powerful than he is.
The duel
! <PERSON> was on the defensive and he clearly toyed with her....only for one reason (and despite his terrible wounds). He wanted to train her in the dark side and make her fall. He had her at his mercy until she closed her eyes and did what <PERSON> did during the battle of <PERSON>. She calmed her mind and let the force flow instead of blocking it out. 865
Then she beat him fully. Here we can only speculate how she could beat someone who has way more training than her with the force and the lightsaber than her. Possible is as stated above that she has training that she doesn't remember OR that she is WAY more powerful than him (would also explain how the Great leader and <PERSON> could feel her awakening). Additionally pointing into the WAY more powerful direction is the fact that <PERSON> tried at the beginning of THEIR duel to telekinetically call the lightsaber........and it flew but not to him but to <PERSON>.
A bit here additionally (personal points):
! In addition though is what I personally find confusing about the duel. The stormtrooper who seemingly is NOT force sensitive easily shot <PERSON> first and then attacked him. Even with his wounds <PERSON> should have easily been able to vanquish him without even getting a scratch. Then <PERSON> beats him easily as soon as she does not act like she wants but lets the force flow. So aside from the two possibilities above there could be another one that <PERSON> is just not as powerful as he wants to be. That is coutnered though by his blaster freezing in the beginning of the film. So he IS powerful. And she has to be stronger as it seems. One thing we know for sure though is that <PERSON> is NOT as powerful as <PERSON> was (yet). As it is still his fear that he never becomes THAT powerful. 865