The degree of neuroplasticity and age is currently being studied and not fully understood. We do know that at a very young age, the brain is capable of some serious remodeling to circumvent injured areas. The question is at what age does this begin to decline and how can we take advantage of this phenomenon in non-pediatric patient populations. From what we are seeing currently, there isn't much difference in a 30 year old versus a 50 year old. What does happen is that the likelihood of vascular secondary injury increases with age as the brain shrinks inside the cranial vault, thus leading to more shearing injury to vasculature in the areas between the brain and the three layers of mater (dura, arachnoid, pia) and the skull. This isn't a direct neurologic injury from the impact, which the helmet would have absorbed. It is caused by the brain slushing around inside the cranial vault excessively in an otherwise non-life-threatening G load to the skull.
Disclaimer: I have a two week old baby right now and am totally sleep deprived. Not sure if any of this is making sense.