The most preclinical used in vivo model to study lung fibrosis is the bleomycin-induced lung fibrosis model in 2-3-month-old mice. Although this model resembles key aspects of idiopathic pulmonary fibrosis (IPF), there are limitations in its predictability for the human disease. One of the main differences is the juvenile age of animals that are usually used in experiments, resembling humans of around 20 years. Because IPF patients are usually older than 60 years, aging appears to play an important role in the pathogenesis of lung fibrosis. We here compared young (3 months) and old (21 months) mice 21 days after intratracheal bleomycin instillation.