Thermodynamically, our peak was at conception. From there we started the process of dying. Corruption and chaos is where life and all that we create with our hands will end. We may be able to extend our life by some time with proper care, but all ends.
But, we are NOT just physical. There is no location for consciousness in our brain. The brain is the emergent construct of consciousness. We are spirits that interact with the physical world through the brain and the body it controls and receives information from.
When someone has brain damage or insufficient capacity, then the spirit that connects through it does not receive accurate information to understand the physical world accurately. The reaction to the skewed information may return with what most would see as a flawed response.