It is believed that second language acquisition is "harder" than first language for a number of reasons, such as the placidity of the brain crystallising as we

It is believed that second language acquisition is "harder" than first language for a number of reasons, such as the placidity of the brain crystallising as we age and, potentially, that any special hardware we may have has a finite capacity (because physics). People research this (my wife is doing her PhD on it currently) and the evidence is neither obvious nor conclusive.