This article focuses on grammar, and the conclusions there seem plausible to me. But a related claim is that it's much harder to lose a foreign accent when speaking if exposed to the language past age 13 or so, and there I'm leaning toward thinking that this is true.

In reading and writing, I'm much better in English than in French. I bet I could pass for a native writer in the former (or fail to do so primarily because my spelling is TOO accurate ;-). But when speaking, I have a pretty obvious accent in English, despite spending years being immersed in it, and working on my pronunciation.

My spoken French, by virtue of starting a few years earlier (and being mercilessly drilled on pronunciation), is better, despite never having reached as high a level of immersion, and having lost quite a bit of my vocabulary.