The answer to the question is far from settled. While there are many similarities
between L1 and L2 learning, the variation in situation and other factors
also produces many differences. One difficulty is filtering out differences that
are accidental rather than inevitable. L1 children mostly acquire language in
different settings with different exposure to language than L2 learners and
they are at different stages of mental and social maturity (Cook 1969). It may
be inherently impossible to compare equivalent L1 and L2 learners. A more
precise version of this question asks whether adults still have access to Universal
Grammar in the mind, to be discussed below.