During the first months of the SARS-CoV-2 epidemic in 2020, Spain implemented an initial lockdown period on March 15 followed by a strengthened lockdown period on March 30 when only essential workers continued to commute to work. However, little is known about the epidemic dynamics in different age groups during these periods. We used the daily number of COVID-19 cases (by date of symptom onset) reported to the National Epidemiological Surveillance Network (RENAVE) among individuals aged 15-19y through 65-69y. For each age group g, we computed the proportion PrE(g) of individuals in age group g among all reported cases aged 15-69y during the pre-lockdown period (March 1-10, 2020) and the corresponding proportion PrL(g) during two lockdown periods (initial: 25 March-3 April; strengthened: 8-17 April, 2020). For each lockdown period, we computed the proportion ratios PR(g)= PrL(g)/PrE(g). For each pair of age groups g1,g, PR(g)>PR(g) implies a relative increase in the incidence of detected SARS-CoV-2 infection in the age group g compared with g for the lockdown period vs. the pre-lockdown period. For the initial lockdown period, the highest PR values were in age groups 50-54y (PR=1.21; 95% CI: 1.12,1.30) and 55-59y (PR=1.19; 1.11,1.27). For the second lockdown period, the highest PR values were in age groups 15-19y (PR=1.26; 0.95,1.68) and 50-54y (PR=1.20; 1.09,1.31). Our results suggest that different outbreak control measures led to different changes in the relative incidence by age group. During the initial lockdown period, when non-essential work was allowed, individuals aged 40-64y, particularly those aged 50-59y, had a higher relative incidence compared with the pre-lockdown period. Younger adults/older adolescents had an increased relative incidence during the later, strengthened lockdown. The role of different age groups during the epidemic should be considered when implementing future mitigation efforts.