Danish Mehboob

(CNN Money) -- Pay inequality between men and women isn't just a problem for your parents' generation.

Of the highest-paid executives age 40 and under last year, just one woman cracked the top 20, according to an analysis by Equilar, an executive and board data provider.

Julia Haywood came in at No. 19. Haywood earned $3.9 million when she was vice president of United Continental Holdings, which operates United Airlines. She left that post in January 2017 to return to her former company, Boston Consulting Group.

The highest-paid executive on Equilar's "40 and under" list was Extraction Oil & Gas president Matthew Owens, who earned $26 million. Equilar's list is populated using SEC records from the 2016 fiscal year.

Much of the difference between the two executives' pay came from factors like stock awards and bonuses. Haywood earned $2.4 million in stock awards, while Owens earned $14.9 million.

Part of the reason more women aren't making it to high-paying executive positions is because the corporate world is still governed by old-school rules, experts say.

"Even if recent generations may have very different attitudes, frequently the people that decide about their careers and promotions belong to an earlier generation," says Luca Flabbi, an associate professor of economics at the University of North Carolina.

While men and women seek promotions at similar rates, women are less likely to be promoted than their male counterparts. That's according to a report by McKinsey & Company, which surveyed 222 companies across the tech, finance, retail and manufacturing industries.

In addition, women who negotiated for promotions were 30% more likely than men to be labeled as "intimidating" or "bossy," McKinsey found.

According to Lenora Lapidus, director of the ACLU Women's Rights Project, women face a pay gap at all ages, but it only grows with time and experience. That's especially true in the business and tech industries, and most of the executives represented on Equilar's list work in tech. Still, the pay gap is a problem across industries, with energy and real estate also heavily represented.

"Business school graduates have similar earnings at the beginning of their careers," says Mario Marcis, an economics professor at Johns Hopkins. "But 15 years after graduation, men are earning a lot more than women."

Experts believe that talking about workplace inequality is one way to help close the pay gap for new generations.

"20 years from now, I expect to see more women CEOs in the United States." said Marcis. "But it depends on the speed of change of social norms about gender roles. Social norms tend to change slowly."