However, some character traits like having high aspirations for yourself increased with age, said the team from Michigan State University (MSU).
"There's a narrative in our culture that generations are getting more and more narcissistic, but no one has ever looked at it throughout generations or how it varies with age at the same time," said William Chopik, associate professor of psychology at MSU and lead author.
The research, published in Psychology and Aging, assessed a sample of nearly 750 people to see how narcissism changed from age 13 to 70.
The findings showed that narcissism decline over time and with age.
There are things that happen in life that can shake people a little bit and force them to adapt their narcissistic qualities.
"As you age, you form new relationships, have new experiences, start a family and so on. All of these factors make someone realize that it's not 'all about them.' And, the older you get, the more you think about the world that you may leave behind," Chopik explained.
The greatest impetus for declining narcissism was landing a first job.
"One thing about narcissists is that they're not open to criticism. When life happens and you're forced to accept feedback, break up with someone or have tragedy strike, you might need to adjust to understanding that you're not as awesome as you once thought," Chopik said.
"There's a sense in which narcissists start to realize that being the way they are isn't smart if they want to have friends or meaningful relationships."
Chopik found that the fastest-changing age group was young adults.
Contrary to popular belief, changes in the levels of narcissism are lifelong and changes don't stop at any certain age or stage in life.
"One of the most surprising findings was that - also contrary to what many people think - individuals who were born earlier in the century started off with higher levels of hyper-sensitivity, or the type of narcissism where people are full of themselves, as well as willfulness, which is the tendency to impose opinions on others," Chopik elaborated.
If you're worried that someone is truly a narcissist, there's hope they will change for the better as they get older, the team noted.
( With inputs from IANS )