Structural priming, the tendency to repeat previously uttered syntactic structures, can give insight into human language processing and acquisition. We report two corpus-based studies of children's structural priming that test the following claim of the item-based account of language acquisition: as older children generalize over structures, priming increases with age. A hypothesis derived from this claim, viz., that the lexical boost effect decreases with age, is also tested. We fit mixed-effects logistic regression models on data from children aged 2 to 7.5 years from the CHILDES corpus. We demonstrate structural priming of arbitrary syntactic structures for the first time in child language data. We also find evidence that priming increases with age, but fail to confirm the hypothesis that the lexical boost effect decreases with age.