Mental capacity is your ability to reason, plan and problem solve, the use of language, and to learn. This is just one part of your total intelligence. As with the physical human body your brain can be worked like and muscle making it stronger. One way to strengthen the brain is by putting yourself just outside your comfort zone. Putting yourself and your brain in different kinds of setting that are not the norm allows your brain to strain a little and grow stronger.
There are a few things you can do to help this process in everyday life. Next time you're hungry try a different kind of food you have not tried yet. Just a simple tweak in the day can make a huge impact on you. The list of things you can do is endless and varies with every person. That is what makes the human brain one of the most amazing creations ever. All humans are genetically and physically the same for the most part but yet our brains are one in a million.
Some other things you can do if you feel you have hit a wall, go back to school, and learn to play an instrument, or simply just do something that stimulates your brain. Doing things that you enjoy is a huge benefit for obvious reasons. When doing an activity you enjoy your brain is rewarded and you grow mentally from that. A bad influence would be watching television. since watching it does not use your capacity and it does not let you regain the mental energy. If you are going to watch television watch something educational or something that will make you think. Use of the brain instead of just letting it vegetate will benefit you in the short term and long term of your life.
As we all get older we tend to forget about our minds.