He said that he didn't take (moral) advice from 16 year olds.
An extended quote below, in case the link target disappears
On your most recent Fireside Chat, you said that people are not basically good. We’ve heard you discuss this topic before. Anne Frank is quoted as saying, ‘Despite everything, I believe that people are really good at heart.’ How do you respond to her quote?”
Here is my response (this is a word-for-word transcription, except for the words in parentheses added for clarity): “She wrote that in her diary, the most famous Holocaust document. (She was) a teenage girl, a Dutch Jewish girl, who hid with her family until they were betrayed by someone to the Nazis, who then shipped them to death camps. And she died, murdered by the Nazis in the death camps. She was about 16 years old, maybe 15. Her diary is very famous. It gives a face to the horror of the Holocaust.
“I know she wrote that, and my answer is it doesn’t matter that she wrote it. I don’t get my wisdom from teenagers. That she was a wonderful young woman and wrote an unbelievably powerful document that will last forever is beside the point. I don’t expect 16-year-olds, unless they grew up in a religious Jewish or Christian home (where it is taught as basic religious doctrine that people are not born basically good). She was a secular Jew. Most kids believe that (people are basically good). But it is not true. So, it has never been an issue for me — ‘Well, you disagree with Anne Frank.’ So what?And Prager is right: along with Jesus (Matt 15:19) "For out of the heart come evil thoughts—murder, adultery, sexual immorality, theft, false testimony, slander."
and Jeremiah (17:9) "The heart is deceitful above all things and beyond cure. Who can understand it?"
The question for anyone, indeed, for oneself, is what are you doing about the evil in your own heart? Best answer is 'turning to Christ in repentance'. Worst answer is 'trying hard.'
Guess which one is the most effective, and which is the most popular.