When can a child start learning L3?

According to research studies and conventional wisdom, children can acquire language effortlessly in comparison to adults despite those children's lack of cognitive and analytical skills,  from their infanthood through adolescence with varying degrees. A child can learn any second language from an early age, maybe2 or3, throughout his/her life. During this period, the language input (Krashen) is fine -tuned - motherese- to them that they can transfer it to uptake and then learning. Also, they have got great brain capabilities that enable their natural processing. However, they still need the social interaction ( Vygotsky) to apply their acquired cognitive and motor skills in L2 learning. In fact, children in their early age are more than willing to interact and negotiate meaning with elders, which pushes them through the threshold of language learning.