The current study examined how morpho-semantic processing of derivational morphology develops from later childhood through adolescence to adulthood in Finnish. Finnish is a synthetic language rich both in derivation and inflection. It has been suggested that children gradually acquire the ability to process morphologically complex word structures. However, this development could be delayed because of the complex derivational morphology in Finnish. To assess this, three age groups of Finnish native speakers participated in a priming study, in which they made a visual lexical decision for the target words. There were three types of primes: morphologically related words, pseudowords, and unrelated words. The reaction times results showed a significant difference between all the groups, which implies that the word processing is still developing in adolescence. The error rates unveiled a similar pattern to reaction times. The prime type affected the recognition: words with morphological primes were processed faster than words with pseudoword primes, which in turn were processed faster than words with unrelated primes. Moreover, males made significantly more errors than females with morphological and pseudoword prime conditions.