Abstract In this study, age of onset (AoO) was investigated in five- and six-year-old bilingual Frisian–Dutch children. AoO to Dutch ranged between zero and four and had a positive effect on Dutch receptive vocabulary size, but hardly influenced the children's accurate use of Dutch inflection. The influence of AoO on vocabulary was more prominent than the influence of exposure. Regarding inflection, the reverse was found. Accuracy at using Frisian inflection emerged as a significant predictor; this transfer effect was modulated by lexical overlap between the two languages. This study shows that ‘the sooner the better’ does not necessarily hold for language development. In fact, for the correct use of inflection, it does not matter whether children start at age zero or four. For rapidly learning words in a new language it may be helpful to first build a substantial vocabulary in the first language before learning a new language.