Your Brain: The Missing Manual

Old Age

Your brain develops at a breakneck speed throughout your early years. From an evolutionary point of view, this makes perfect sense—after all, our ancestors needed every advantage they could get to survive the harsh and brutal prehistoric world long enough to have babies. Unfortunately, once you've passed your genes along you've ensured your evolutionary success, and your personal survival is decidedly less important.

Although we're used to associating old age with the very last decades of life, you may be closer to it than you think. If you've passed the age of 20, your brain has begun its long and steady decline. Your family of neurons, which you've held since birth, is beginning to show some serious wear and tear. As each year passes, your brain shrinks a bit more.

Here's how your brain changes as it ages past 20:

• The brain shrinks. Brain size peaks at 20, and if you reach 100 you may have made it there with 15% less brain. The actual cause for the shrinkage is controversial. Some suggest it's neuron loss, others point to the breakdown of myelin around neurons, while others think it's the result of continued synaptic pruning.

• The brain slows. As we age, our reaction times slow down. When given problems, we reason more slowly and take more time to assemble a plan. Already at 30, neuroscientists can measure performance differences between our current performance and our better 20-year-old selves. Recall is slower and information lingers in short-term memory for a little less ...