That's just the abstract. You apparently didn't read the article, since that pattern is addressed:

sensitive and the researchers found hypersensitivity sharply declined after a person turned 40.

But when the researchers looked at generation-specific trends, they noticed that overall, older generations were more sensitive than younger generations.

A co-author is quoted in the article giving a possible reason by baby boomers are more narcissistic. Here's an excerpt from the study:

That later-born birth-cohorts were lower in hypersensitivity and higher in autonomy was also perplexing as it goes against a narrative that recent birth-cohorts have experienced a monotonic rise in narcissistic traits across the past century