He probably can't remember for sure. I'm years younger than NF and I can't remember half the stuff at school that we did or said. I appreciate the people who he allegedly said it to would remember though. But I think their motives in bringing it up are political so can't really be relied on 100%. I think that's a fair and reasonable approach.
If they can prove he did say those things then that's different as they are awful.
And if they are proven, then the issue of whether you should be held professionally responsible for something you said as a child needs to be considered and what precedent that sets. Bearing in mind it would apply to any politician of any stripe.