Age of acquisition effects or effects of bilingualism in second language ultimate attainment?

One of the most robust findings in the field of SLA is the different rates of success with which children and adults achieve nativelike proficiency in a L2. Age-related differences have traditionally been explained in terms of the maturational state of the learner. Recently, however, a growing number of accounts hold that age effects in ultimate attainment are due to L1 entrenchment (e.g. Flege 1999; MacWhinney 2005; Ventureyra, Pallier & Yoo 2004). In this view, an increase in L1 proficiency leads to the progressive entrenchment of L1 representations, with the consequence that L2 acquisition becomes more difficult. Inherent in this interpretation is the assumption that the “less L1”, the less it will interfere with the L2. In this paper, we analyse the theoretical underpinnings of the “L1 entrenchment accounts”, and evaluate the existing evidence for and against such claims.