Background: Alemtuzumab significantly improved clinical and MRI outcomes vs. subcutaneous interferon beta-1a (SC IFNB-1a) in the CARE-MS trials (NCT00530348, NCT00548405), with sustained efficacy in 2 consecutive extensions (NCT00930553, NCT02255656 [TOPAZ]). Methods: Post hoc analysis of 8-year alemtuzumab efficacy and safety in pooled CARE-MS patients (N=811) stratified by baseline age (≥18 to ≤25, >25 to ≤35, >35 to ≤45, >45 to ≤55 years). Results: Compared with SC IFNB-1a over 2 years across age cohorts, alemtuzumab lowered annualized relapse rates (ARR; 0.22–0.24 vs. 0.38–0.51), improved or stabilized disability (freedom from 6-month confirmed disability worsening [CDW]: 85%–92% vs. 62%–88%; achievement of 6-month confirmed disability improvement [CDI]: 20%–31% vs. 13%–25%), increased proportions free of MRI disease activity (70%–86% vs. 42%–63% per year), and slowed brain volume loss (BVL; –0.45% to –0.87% vs. –0.50% to –1.39%). Through Year 2, the treatment effect with alemtuzumab did not significantly differ among age groups for ARR (p-interaction=0.6325), 6-month CDW-free (p-interaction=0.4959), 6-month CDI (p-interaction=0.9268), MRI disease activity-free (p-interaction=0.6512), and BVL (p-interaction=0.4970). Alemtuzumab remained effective on outcomes through Year 8 across age groups. Age-related increases in malignancies (≤45 years: 0.9%–2.2% vs. >45 years: 8.1%) and deaths (0%–1.7% vs. 7.0%) were observed. Serious infections also increased from the youngest (5.1%) to oldest (12.8%) age cohorts. Conclusions: Alemtuzumab had greater efficacy than SC IFNB-1a over 2 years across comparable age groups, with no significant differences between alemtuzumab-treated age groups. Efficacy on relapse, disability, and MRI outcomes continued through Year 8 across age groups. Age-related increases in serious infections, malignancies, and deaths were observed.