Purpose: The purpose of this study is to test whether age-related differences in grammatical word production are due to differences in how children and adults chunk speech for output or to immature articulatory timing control in children. Method: Two groups of 12 children, 5 and 8 years old, and 1 group of 12 adults produced sentences with phrase-medial determiners. Preceding verbs were varied to create different metrical contexts for chunking the determiner with an adjacent content word. Following noun onsets were varied to assess the coherence of determiner-noun sequences. Determiner vowel duration, amplitude, and formant frequencies were measured. Results: Children produced significantly longer and louder determiners than adults regardless of metrical context. The effect of noun onset on F1 was stronger in children's speech than in adults' speech; the effect of noun onset on F2 was stronger in adults' speech than in children's. Effects of metrical context on anticipatory formant patterns were more evident in children's speech than in adults' speech. Conclusion: The results suggest that both immature articulatory timing control and age-related differences in how chunks are accessed or planned influence grammatical word production in school-aged children's speech. Future work will focus on the development of long-distance coarticulation to reveal the evolution of speech plan structure over time.