Idioms, along with other formulaic multiword phrases, represent a substantial part of vocabulary knowledge. This study investigates how idiom knowledge develops through the adult lifespan, comparing familiarity and transparency ratings for a large set of common English idioms. A total of 237 participants, ranging from 18 to 77 years old, collectively rated 200 idioms. They also completed a short single-word vocabulary test and provided information about their educational background. Results showed a clear increase in idiom and single-word knowledge throughout the lifespan. For idioms, this represented a jump from the youngest age-group, then a steady increase from the age of around 25 onward. Single word vocabulary knowledge increased more evenly as a function of age. Perceptions of transparency were not affected in the same way. I discuss what these results suggest about the development of vocabulary through the lifespan.