When synapses are working correctly, they allow your neurons to communicate with each other, which keeps your nervous system functioning the way it should. Your nervous system must function properly in order for you to learn new things, retain information, and use your powers of logic and reason.
You're feeling some love for your synapses now, aren't you?You have about 100 billion neurons in your brain, but each neuron has multiple synapses connecting to multiple other neurons. As a result, you have literally trillions of synapses — possibly even a quadrillion (that's a 1 followed by 15 zeroes). It sure seems like you have plenty to spare, but as you age, your synapses deteriorate. And because your brain activity takes place courtesy of synapses, their deterioration equates to a decrease in your brain function, including memory.
The take-home point here is that if you want your mind to stay sharp to a ripe old age, you need to do more than just take care of your body (although that's crucial too). You need to keep your synapses in top condition.