@Mark Roy: 

In general, younger learners are more likely to struggle and strand in speaking and writing (productive side) while older learners would be liable being stalled in listening and reading (receptive side). Surely that is a very rough rule of thumb. 

@ Yi Zhang
I met a couple of IELTs candidates who did not get good mark in speaking are actually fluent in conversation. I guess that is because in the test the candidates have to use well-structured compound sentences to describe an abstract, whereas that is seldom occurring in everyday conversation with your neighbors. That is why many candidates think speaking module in the test is much more difficult than other skills. 

@Jack

Yes, I agree. It is the same case with me. After a few years of effort, many of us speak fluently in the most of the everyday life scenarios.  However, those scenarios do not necessarily apply to the language exams. Instead, questions in IELTS speaking task are those we normally would not think about in depth. The simplest way to test is to answer those questions in our first language. To be honest, I do not think I can answer some of those tricky questions, like what is my opinion on rock music, regardless in Chinese or English, especially we have to elaborate our answers within such short period.