This study examines the impact of L2 literacy on the development of writing proficiency in the L3, as related to age of onset (AO) of instruction, as well as the effects of AO on ultimate L3 attainment at the end of the period of normal schooling. Using longitudinal data for the same student cohort (200 Swiss learners of English) at the beginning and at the end of secondary education, it was found that advantages of an early start do not emerge even after a substantial amount of input has been gained. The late starters, who began English instruction five years after the early starters, were able to catch-up to the written performance of the early starters within six months, and this remained true even when the total number of hours of study input was not controlled for. Quantitative analysis revealed that this may be explained in terms of a possible "threshold level" for L2 writing to transfer to the L3, since the late starters began L3 education with a better foundation in L2 German, which is their primary language of literacy. Furthermore, high statistical correlations were found between L2 and L3 literacy skills irrespective of starting age.