In the book, they were of similar age. I think the article is more about the Hollywood habit of casting young female leads to star opposite older leading men because of hotness reasons, not because the characters themselves were inappropriate. Laura Dern was 23 and her character was the co-leader of an archeological dig, which would require like, at least a master's degree if not a PhD.
FWIW, had no clue she was 23 in JP. I thought early to mid 30s just because of the expertise her character was portraying. And unlike, say, a Megan Fox, who was 20 in the first Transformers film, Laura Dern wasn't just some sexy sideplot.