Background: Children and adolescents are affected in various ways by the lockdown measures due to the COVID-19 pandemic. Therefore, it is crucial to better understand the effects of the COVID-19 pandemic on mental health in this age-group. Objective: The objective was to investigate and compare the effects of the COVID-19 pandemic on mental health in three age groups (1-6 years, 7-10 years, 11-19 years) and to examine the associations with psychological factors. Methods: An anonymous online survey was conducted from 9 April to 11 May 2020 during the acute phase of major lockdown measures. In this cross-sectional study, children and adolescents aged between 1 and 19 years were recruited as a population-based sample. They were eligible if they were residents in Austria, Germany, Liechtenstein or Switzerland, were parents/caregivers of a child aged between 1 and 10 years or adolescents ≥11 years, had sufficient German language skills and provided informed consent. Results: Among 5823 participants, between 2.2% and 9.9% reported emotional and behavioural problems above the clinical cut-off and between 15.3% and 43.0% reported an increase in these problems during the pandemic. Significant age-related effects were found regarding the type and frequency of problems (χ 2 (4) ≥50.2, P ≤ 0.001). While preschoolers (1-6 years) had the largest increase in oppositional-defiant behaviours, adolescents reported the largest increase in emotional problems. Adolescents experienced a significantly larger decrease in emotional and behavioural problems than both preschoolers and school-children. Sociodemographic variables, exposure to and appraisal of COVID-19, psychotherapy before COVID-19 and parental mental health significantly predicted change in problem-scores (F ≥ 3.69, P ≤ 0.001). Conclusion:A substantial proportion of children and adolescents experience age-related mental health problems during the COVID-19 pandemic. These problems should be monitored, and support should be offered to risk-groups to improve communication, emotion regulation and appraisal style. Efectos de la pandemia de COVID-19 relacionados con la edad en la salud mental de niños y adolescentesAntecedentes: Los niños y adolescentes se ven afectados de diversas maneras por las medidas de confinamiento debidas a la pandemia de COVID-19. Por lo tanto, es crucial comprender mejor los efectos de la pandemia de COVID-19 en la salud mental de este grupo de edad. Objetivo: El objetivo fue investigar y comparar los efectos de la pandemia COVID-19 en la salud mental en tres grupos de edad (1-6 años, 7-10 años, 11-19 años) y examinar las asociaciones con factores psicológicos. Métodos: Se realizó una encuesta anónima en línea del 9 de abril al 11 de mayo de 2020 durante la fase aguda de las principales medidas de confinamiento. Como una muestra basada en la población, en este estudio transversal, se reclutaron niños y adolescentes de entre 1 y 19 años. Eran seleccionables si residían en Austria, Alemania, Liechtenstein o Suiza, si eran padres/cuidadores de un niño de e...