When one of my children was 7 years old, the school required some use of the internet by the students for assignments and for remote learning. We also started the kids on taking classes by remote learning outside of school even earlier. None of this had any ill effects that we could observe.
We have been avoiding social networks, as these are documented to be harmful for children in multiple research publications (see the links below).
I am not aware that the optimal age of introduction to the Internet, per se, has been extensively studied. By contrast, the general effects of video games, social media and screen time on children in general are better studied, but these are outside of the scope of this question.
References:
Social media studies:
OVERALL CONCLUSION: Many studies, using a variety of methods, have found associations between heavy social media use and bad mental health outcomes, particularly for girls. Some of the associations are very small, some are larger (e.g., a doubling of rates of depression as one moves from light to heavy usage in 1.1.4, Kelly et al. 2019; a large decline in depressive symptoms when college students were assigned to reduce social media usage in 3.1.1, Hunt et al. 2018). The recent publication of two papers that find no effect (2.2.1, Heffer et al. 2019), or negligible effects (1.2.1, Orben & Przybylski, 2019) is a normal part of the ongoing scientific debate about the effects of social media on teen mental health. We believe that journalists, legislators, parents, and teens would be making a potentially serious mistake if they interpret the minority of studies that find negligible or null effects as offering an “all clear” signal for teens to use social media in unlimited quantities, or from an early age. But we welcome feedback from researchers who disagree, and we will post short response essays below.
Haidt, J., Rausch, Z., & Twenge, J. (ongoing). Social media and mental health: A collaborative review. Unpublished manuscript, New York University. Accessed at tinyurl.com/SocialMediaMentalHealthReview
In particular: among girls, there is a consistent and substantial association between mental health and social media use (median betas from −0.11 to −0.24). These associations were stronger than links between mental health and binge drinking, sexual assault, obesity, and hard drug use, suggesting that these associations may have substantial practical significance as many countries are experiencing rising rates of depression, anxiety, and suicide among teenagers and young adults.
Twenge JM, Haidt J, Lozano J, Cummins KM. Specification curve analysis shows that social media use is linked to poor mental health, especially among girls. Acta Psychol (Amst). 2022 Apr;224:103512. doi: 10.1016/j.actpsy.2022.103512. Epub 2022 Jan 29. PMID: 35101738: PubMed
Internet use observational studies (not well-controlled):
In our study, children's Internet use was associated with obesity, psychological disturbances and social maladjustment.
Novaković S, Milenković S, Srećković M, Backović D, Ignjatović V, Capo N, Stojanović T, Vukomanović V, Sekulić M, Gavrilović J, Vuleta K, Ignjatović V. Children's Internet use and physical and psychosocial development. Front Public Health. 2023 Jun 8;11:1163458. doi: 10.3389/fpubh.2023.1163458. PMID: 37361154; PMCID: PMC10285096.: PubMed
Gender, age, digital media usage time and the intensity of negative emotions during the COVID-19 pandemic were all found to be significant predictors of Problematic Internet Use (PIU): female gender, increasing age, longer digital media usage time and higher intensity of negative emotions during the COVID-19 pandemic were associated with higher Short Compulsive Internet Use Scale (SCIUS) total scores. This study found a very high prevalence of PIU among 12- to 17-year-olds for the period after the third wave of the COVID-19 pandemic, which has increased significantly compared to pre-pandemic prevalence rates.
Paulus FW, Joas J, Gerstner I, Kühn A, Wenning M, Gehrke T, Burckhart H, Richter U, Nonnenmacher A, Zemlin M, Lücke T, Brinkmann F, Rothoeft T, Lehr T, Möhler E. Problematic Internet Use among Adolescents 18 Months after the Onset of the COVID-19 Pandemic. Children (Basel). 2022 Nov 10;9(11):1724. doi: 10.3390/children9111724. PMID: 36360452; PMCID: PMC9689314: PubMed