A book I read about the brain and brain surgery explains why older people are less likely to get concussions or damage than teens and young adults. Leaving aside risky behavior, it’s because your brain atrophies beginning in your 20s.

Thus, if a brain injury with swelling in a teen occurs, the skull is pretty full and the pressure occurs quickly, with possible problems with blood flow. That older brain takes up less volume, and thus some swelling can be accommodated without squeezing against the skull in many cases.