I have observed several children acquiring a first language. They struggle with pronunciation, repeat themselves again and again trying to get words right, mimic, ask for help, and generally work their little butts off. There's nothing effortless about it.
I've also seen many adults acquire second languages and fail to do so. I have also learned to speak a second language fluently as an adult myself.
My hypothesis is that adults' greater difficulty in learning a language is mostly based in fear of failure, social embarrassment, the vulnerability of asking for help, and the necessary process of making a fool of yourself along the way. Children are unafraid to blunder repeatedly until they get it right.
Wikipedia distinguishes between first and second language acquisition (FLA, SLA). FLA is done by children. SLA is done by children and adults.
The critical period hypothesis is that SLA is age dependent and can only be done properly as a child. However "The critical period hypothesis is the subject of a long-standing debate in linguistics and language acquisition over the extent to which the ability to acquire language is biologically linked to age."
I am not familiar with the research in this field, but I don't think it is clear cut that SLA is age dependent. FLA does appear to be though.
FLA is most definitely age dependent. There have have been a couple of real sad cases where this has been tested - the case of 'Genie' is the most famous[0].
I've also seen many adults acquire second languages and fail to do so. I have also learned to speak a second language fluently as an adult myself.
My hypothesis is that adults' greater difficulty in learning a language is mostly based in fear of failure, social embarrassment, the vulnerability of asking for help, and the necessary process of making a fool of yourself along the way. Children are unafraid to blunder repeatedly until they get it right.