what age is it easiest to learn a second language?

Monika Schmid:

addition to this overall and gradual advantage for younger learners, there is one notable qualitative difference: even very good older language learners differ from younger ones when it comes to using grammar correctly and consistently. Every time I mark a run of scripts from my adult students, most of whom are from non-English-speaking backgrounds, I find that while they are amazingly good at using a wide range of vocabulary, appropriate style and complex grammar, they often struggle with some simple grammatical rules.

For example, many adult learners never fully master the distinction between “he walks” and “they walk”. They also often fail to grasp that “I have lived in Colchester for two years” means that I still live there, while “I lived in Colchester for two years” means that I do not. Why are simple and highly frequent rules apparently impossible to master, while words that have been encountered only a few times sink in easily?