This study examined to what extent advanced and beginning readers, including dyslexic readers of Dutch, make use of morphological access units in the reading of polymorphemic words. Therefore, experiments were carried out in which the role of singular root form frequency in reading plural word forms was investigated in a lexical decision task with both adults and children. Twenty-three adult readers, 37 8-year-old children from Grade 3, 43 11-year-old children from Grade 6, and 33 11-year-old dyslexic readers were presented with a lexical decision task in which we contrasted plural word forms with a high versus low frequency of the singular root form. For the adults, it was found that the accuracy and speed of lexical decision is determined by the surface frequency of the plural word form. The frequency of the constituent root form played a role as well, but in the low-frequency plural words only. Furthermore, a strong developmental effect regarding the accuracy and speed of reading plural word forms was found. An effect of plural word form frequency on word identification was evidenced in all groups. The singular root form frequency also had an impact of the reading of the plural word forms. In the normal reading and dyslexic children, plurals with a high-frequency singular root form were read more accurately and faster than plurals with a low singular root frequency. It can be concluded that constituent morphemes have an impact on the reading of polymorphemic words. The results can be explained in the light of a word experience model leaving room for morphological constituency to play a role in the lexical access of complex words as a function of reading skill and experience and word and morpheme frequency.