This inquiry explores L2 literacy influence on L3 writing skill evolution, as linked to instructional age of onset (AO), and the AO impact on final L3 acquisition at the end of the normal schooling period. Using a five-year longitudinal study of Swiss learners of English (N=200) during secondary education, early start gains do not occur even after gain of a notable input amount. Quantitative analysis disclosed that this may be interpreted in terms of an eventual ‘threshold level’ for L2 writing to trasmit to the L3, as the late starters commenced L3 education with a better basis in L2 German, their primary literacy language. Moreover, high statistical links were found between L2 and L3 literacy abilities regardless of starting age. Keywords: age and language learning, bilingual education, EFL, multilingual language acquisition, language policy, language transfer