Background: Insufficient physical activity levels (PAL) during adolescence is a major public health concern, which is even more pronounced during the COVID-19 pandemic due to restricting movement opportunities. This study aimed to identify PAL changes and examine the age-specific determinants of PAL in younger and older adolescents during the COVID-19 lockdown. Material and Methods: This study included 859 high-school students from Bosnia and Herzegovina stratified into 2 age groups: younger (N = 420, 14–16 years of age), and older adolescents (N = 439, 16–18 years of age). Participants were tested over 2 testing waves: before the COVID-19 lockdown (January 2020) and during the COVID-19 lockdown (April 2020). Variables included PAL assessed by the Physical Activity Questionnaire for Adolescents, sports factors, and parental factors. Results: The PAL significantly declined as a result of lockdown measures in the total sample (from 2.76±0.79 to 2.50±0.82). Larger absolute and relative decline of PAL was evidenced in younger adolescents. Sport participation positively influenced PAL before lockdown, with no significant influence during the lockdown. Older adolescents whose mothers were better educated were less likely to be in high risk group with regard to a large decline of PAL as a result of COVID lockdown (OR = 0.50, 95% CI: 0.21–0.84). Conclusions: Results of the study suggest that parental education influences health-related behaviors and that parental education is a protective factor against a decrease in PAL during the COVID-19 pandemic. Main educational agents (i.e., school and parents) should pay more attention to provide children and adolescents adequate information and develop their health literacy, which will hopefully positively impact children’s PAL even in challenging situations similar to COVID-19 lockdown. Med Pr. 2021;72(6):633–43