Photo from the New York Times Health section.
If you didn't already have enough reasons to wear your helmet, this story from the New York Times says that our brains shrink as we age. Therefore, there's extra room inside our skull so that if we fall our brains get slammed around more. Conclusion: if you're older, wear your helmet.
And in case you wondered whether wearing your helmet matters in a crash, statistics confirm you are significantly more likely to suffer serious head injury without a helmet. Conclusion: if you eventually want an older, smaller brain and you're young, wear your helmet now so you can wear your helmet later.