US News article says "The inevitable physical changes start in early adulthood but become especially marked after about age 60 or so. Gradually, the brain shrinks, losing around 0.5 percent to 1 percent of its volume each year after that age threshold; brains with Alzheimer's shrink about twice as fast."

Article in ScienceAGoGo states "everyone's brain starts shrinking beginning at young adulthood. On average, a person's brain will shrink 2.5 percent every 10 years."

But some parts (such as the hippocampus) of the brain shrinks more than other parts. See picture.

In addition to brain mass shrinking, outer surface of the brain thins, brain white matter decreases, and chemical messengers decrease as well.^[ref]

In general, as you age, you have fewer brain cells than when you are in your youth.

Article in AOL Health reports that "The typical adult brain does lose neurons with age".

Reversing Memory Loss states "as we grow older we have fewer brain cells".^{[Page 133]}

The article "Higher Stress Hormone Correlated With Shrinkage In Brain Region" cites a study that shows higher level of stress hormone is correlated with brain shrinkage.

Dr. Khalsa says that cortisol (our stress hormone) kills brain cells by the millions.  An article on thinkquest.org says "As one ages, hippocampus in the limbic system of the brain is damaged from 20 - 25%. ... hippocampus is most damaged by the production of the cortisol."

BBC News reports "Stressful event kills brain cells".

However, there are other reasons why a brain might shrink.

Article in CalorieLab cites some other reasons why our brains shrinks -- some of which are excessive alcohol, cannabis smoking^[ref], lack of B12^[ref], and obesity^[ref].

A study reported in PsychCentral "found that the more alcohol consumed, the smaller the total brain volume over time."

The good news is that one study found that "on average, men aged 64 years have smaller brains than men aged 60. However, despite this shrinkage, cognitive functions - like memory, attention and speed of processing - are unaffected."

Exercise is an excellent way to encourage the process of neurogensis. John Ratey, author of Spark: The Revolutionary New Science of Exercise and the Brain, says that "exercise is the single best thing you can do for your brain" ^[ref]

If you need some motivational techniques to get yourself to exercise, see article linked here.

A study published in the journal Neurology found that B12 have protect against brain volume loss.^[ref]