While the brain changes with age, not all changes mean that the brain is moving towards a dementia, like Alzheimer’s disease.
In general, as people age BRAIN MASS SHRINKS. This is normal and expected for people beginning in their 60’s or 70’s.
Second, the OUTER SURFACE or CORTEX of the brain begins to get thinner. Since that is where connections for communication is located, the ability to form new connections decreases (although this is a process that begins in the 20’s).

Third, the WHITE MATTER of the brain, consisting of a substance called myelin, decreases. This involves changes in the speed of cognitive processing, although ti does not affect the store of knowledge accumulated over time.
Lastly, CHEMICAL MESSNGERS, the neurotransmitters (like serotonin) that carry messages between brain cells, slow down. This is because there are less of them, and less of an opportunity that they will latch onto receptor sites to store newly acquired information. This change may have an effect on memory.