This study analyses the emergence of the noun plural category in typically developing Danish-speaking children from its first appearance up to the age of 10 years, focusing on the impact of sound structure and input frequency. We use a multi-method research approach comparing different data types (dictionary data, naturalistic spontaneous child language input and output, semi-naturalistic/semi-experimental data, experimental data and reported data). We define cross-linguistically three degrees of stem changes (no change, prosodic change, phonemic change), and we also define three degrees of productivity of plural markers (which combine stem change and suffix). Noun plurals emerge from an early age, typically around the second birthday, but the acquisition is still underway at the age of ten years. Plural acquisition is affected by frequency and morphophonological category. Danish children produce more correct plural stems of nouns with non-changing plural stems compared to plural nouns with stem change, and more correct plural stems of nouns with prosodic change than with phonemic change. Furthermore, they produce more correct plural suffixes of nouns with a-schwa suffix than with e-schwa and zero suffix. Danish children also produce more correct plural forms of nouns with a fully productive than a semi-productive plural marker, and more of the latter than of nouns with an unproductive plural marker. We also discuss the important role in Danish of the plural marker ø, where pl = sg.