Meghan Roark isn’t too proud to admit she has an addiction. Her habit? Makeup.

Ms. Roark, a 27-year-old who works in retail in Abingdon, Va., estimates that she spends $300 a month on cosmetics and skin care. She watches at least three hours of tutorials each week on YouTube, learning new techniques or keeping up on emerging brands. Her morning makeup routine takes 30 minutes and involves up to 15 products.

Young shoppers like Ms. Roark are the driving force behind a boom in the cosmetics industry. Always camera ready, they are buying and using almost 25 percent more cosmetics than they did just two years ago and significantly more than baby boomers, according to the research firm NPD. And millennials who identify themselves as “makeup enthusiasts,” NPD found, are using six products each day.

Ms. Roark, after setting aside money she had received as a birthday gift, spent $109 during a recent shopping spree at Ulta Beauty, picking up primer, foundation and a new eye shadow palette. “I think every girl likes buying clothes, but for me, I prefer to spend my money on makeup,” she said.

The striking expansion in cosmetics is a bright spot in what is otherwise a challenging environment for retailers and packaged goods companies. Big jumps in the sale of shimmery highlights, lush liquid stain lipsticks and dewy foundations have propelled the stocks of cosmetics giants Estée Lauder and L’Oreal to record highs.