QFE. Recent Neuroplasticity work has overturned a lot of long held scientific beliefs about the brain, such as that adult brains don't grow.
Your brain is like a muscle, the more you work it the more new neurons and connections are formed. It's called neuroplasticity.
The brains of children are the most malleable, and it's easiest to learn things at that stage. A lot of things do get "locked down" somewhat in this initial development period, which is why childhood learning is so important. e.g. if you learn a 2nd language later in life, it will likely be stored in a completely different physical area of your brain than where languages that you learn as a child are stored.
In adolescence/teenage-hood, you brain goes through a period of above-average "pruning", where useful behaviours are cemented and unnecessary cruft is thrown out. This settles down in adulthood, and things settle into a "normal" state. It used to be thought that at this stage, everything was completely finished and locked down, but that's not true; it just takes more practice to learn things in adulthood than it does in childhood.
For an extreme example, stoke or injury victims have lost entire parts of their brain, responsible for things such as their primary language, etc... These people are basically like an adult who somehow never learnt English, and with enough practice, can be taught English (in a different, undamaged, physical area of their brain) up to a similar level that they had before their injury -- something the old school of brain scientists didn't believe was possible.
Just like an obese person deciding to get on a treadmill, it's hard for a long-idle brain to develop new skills, but that just means you've got to be ok with hard work.
If you don't let your brain go idle, and spend your whole life learning new things, then in old age, your brain will weigh a lot more than your idle neighbour's.