It's a bit sad to ponder. On the other hand, the volume of white matter is still rather close to it's maximum, even at 60 years. And probably there are also individual variations, both when it comes to the timing of the onset of the decline, and the rapidity of it.

I read about Stradivarius the other day. His "golden years", the years when the quality of his work was considered to be at it's peak, was from he was 50 until he was around 80.