Collagen is an essential ingredient in dietary supplements for its anti-ageing benefits, and l-cystine-based supplementation has garnered interest for its ability to improve skin condition. Our study aimed to evaluate the effects of oral supplementation combining l-cystine and fish collagen peptides at two different dosages on mature (55–65 years) and young (18–30 years) skin types. Two randomised, three-arm, double-blind, placebo-controlled trials were conducted. A total of 198 Asian women were allocated into equal groups (5.5 or 11 g of active supplement or placebo daily for 12 weeks). In the Young panel, global aesthetic improvement, cutaneous pigmentation homogeneity, texture, redness, acne scars, and UV sensitivity were investigated. In the Mature panel, complexion radiance, crow’s feet, cutaneous thickness, moisturising, pigmentation homogeneity, texture, and skin colour were investigated. In mature women, the most notable improvements after 12 weeks were hydration, cutaneous thickness, and wrinkles. In younger women, improvements in cutaneous texture, reduced skin redness, and increased UV photoprotection were the most notable effects. Oral administration of the dietary supplement containing l-cystine and collagen peptides was perfectly well tolerated. In addition to the already known benefits for mature skin, this supplementation could also benefit younger people with skin imperfections.