Adults over 30 years old are not as happy as they used to be, but teens and young adults are happier than ever, according to a recent study.

American researchers found that after 2010, the age advantage for happiness found in prior research vanished, CBS News reported. There is no longer a positive correlation between age and happiness among adults, and adults older than 30 are no longer significantly happier than those ages 18 to 29.

"Age is supposed to bring happiness and contentment. For that not to be true anymore is somewhat shocking," Jean Twenge, lead author of the study and a professor at San Diego State University, told CBS News.  She also wrote the book "Generation Me," a look at young adults and the attitudes and influences that have helped shape them.

For the study, researchers collected and analyzed data from four nationally representative samples of 1.3 Americans ages 13 to 96 taken from 1972 to 2014, The New York Daily News reported.

Data showed that 38 percent of adults older than 30 said they were "very happy" in the early 1970s, which shrunk to 32 percent in the 2010s. Twenty-eight percent of adults ages 18 to 29 said they were "very happy" in the early 1970s, versus 30 percent in the 2010s.

Over the same time, teens' happiness increased: 19 percent of 12th graders said they were "very happy" in the late 1970s, versus 23 percent in the 2010s.

"American culture has increasingly emphasized high expectations and following your dreams-- things that feel good when you're young," Twenge said. "However, the average mature adult has realized that their dreams might not be fulfilled, and less happiness is the inevitable result. Mature adults in previous eras might not have expected so much, but expectations are now so high they can't be met."

That drop in happiness occurred for both men and women, said Twenge.

"A previous study in 2008 got quite a bit of attention when it found that women's happiness had declined relative to men's. We now find declines in both men's and women's happiness, especially after 2010," she said.

The findings are detailed in the journal Social Psychological and Personality Science.