While attriters are reliably outperformed by native speakers on a range of tasks measuring overall proficiency there is an astonishingly small range of variability and low incidence of non-targetlike use in data even from speakers who claim not to have used their L1 for many decades (in some cases upwards of 60 years, e.g. de Bot & Clyne 1994, Schmid 2002), provided they emigrated after puberty: the most strongly attrited speakers still tend to compare favourably to very advanced L2 learners (Schmid 2009). If, on the other hand, environmental exposure to the L1 ceases before puberty, the L1 system can deteriorate radically.

Academic अकॅडेमिक अभ्यासाचे मूळ
Other studies on bilingualism and attrition
L2
L1 झीज