We investigated age-related decline in adult learning of Dutch as an additional language (Ln) in speaking, writing, listening, and reading proficiency test scores for 56,024 adult immigrants with 50 L1s who came to the Netherlands for study or work. Performance for all four language skills turned out to decline monotonically after an age of arrival of about 25 years, similar to developmental trajectories observed in earlier aging research on additional language learning and in aging research on cognitive abilities. Also, linguistic dissimilarity increased age-related decline across all four language skills, but speaking in particular. We measured linguistic dissimilarity between first languages (L1s = 50) and Dutch (Ln) for morphology, vocabulary, and phonology. Our conclusion is that the L1 language background influences the effects of age-related decline in adult language learning, and that the constraints involved reflect both biological (language learning ability) and experience-based (acquired L1 proficiency) cognitive resources.