Children, especially girls, who own smartphones before they are 13 years old may have worse mental health outcomes when they're older, a new study suggests.

The study, published Sunday in the Journal of Human Development and Capabilities, analyzed self-reported questionnaire results from more than 100,000 young adults between the ages of 18 and 24.

The questionnaire asked respondents about mental health symptoms, such as having aggression, feelings of detachment, hallucinations and suicidal thoughts. Those who were given smartphones at an earlier age were associated with worse mental health outcomes for every year of smartphone ownership before the age of 13.

Early smartphone ownership was associated with feelings of lower self-image and lower self-worth in both girls and boys. Girls reported lower emotional resilience and lower confidence, while boys reported feeling less calm, less stable and less empathetic.

"The younger the child gets a smartphone, the more exposure to all this impacts them psychologically and shapes the way they think and view the world," Tara Thiagarajan, one of the study's authors, told ABC News in an emailed statement.

About 48% of young women who had smartphones by 5 or 6 years old reported having severe suicidal thoughts, compared to 28% of females who had smartphones by 13 or older.

In young men, 31% of those who had smartphones by 5 or 6 years old reported having severe suicidal thoughts and 20% of males who had smartphones by 13 or older reported having severe suicidal thoughts.

Study authors attributed the differences between young women's and young men's mental health symptoms to social media usage. Other factors that seemed to impact mental health outcomes were cyberbullying, poor sleep and poor family relationships.

The study's authors recommended restricting smartphone and social media access for kids under 13, promoting digital literacy education and corporate accountability.

"Ideally, children should not have a smartphone until age 14, and when they do get a smartphone, parents should take the time to discuss with their children how to interact on the Internet and explain the consequences of doing various things," Thiagarajan added.

ABC News' Dr. Tara Narula also said on "Good Morning America" Monday that limiting kids' access to social media appears to be a key step in protecting children and their mental health outcomes.

"The longer we can push off allowing our kids to be on social media, we are learning, the better," Narula said. "I think lots of families are getting creative … landlines …. flip phones for kids [are] maybe an option so that they can have access to communicating without all the other things that come with smartphones."

The study's findings come amid an effort led by social psychologist Jonathan Haidt, author of "Anxious Generation," to limit kids' smartphone use due to the impact on their mental health. Haidt has proposed setting nationwide "norms" or guidelines, including not giving children a smartphone before high school, no social media before age 16 and establishing schools as phone-free zones.

Pediatrician Dr. Natasha Burgert also recommended that parents demonstrate to children how to use smartphones responsibly.

"Children watch everything you do -- and that doesn't stop until they leave your house," Burgert told ABC News via email. "Connect authentically and meaningfully for a few minutes every day, and show your children that the humans we live with are more important and worthy of our attention than our phones."

The American Academy of Pediatrics also recommends families follow the 5 C's of media use, including teaching kids and teens how to be safe online, since content and advertisements may be targeting an older audience.

• Child - Consider your child and their personality. What media are they drawn to and how do they react to it?
• Content - Consider the content of the media your child consumes. Encourage them to consider good media sources.
• Calm - Help your child learn how to manage their emotions, including without the help of media.
• Crowding out - Consider what your family would like to spend more quality time doing, besides consuming media.
• Communication - Discuss media with children early and often and encourage learning digital literacy.