As little as 20 years ago, most people believed that our brain start a slow route to decay shortly after we turn six. According to then, any injury we cause would lead to permanent damage and incapacitation. People understood their mind as something of value, something they had to protect, and people started to say things like: "... the mind is a terrible thing to waste...".
While this is still technically true, we now know that our brain does have stem cells that gives it a regenerative capability, although we have not really seen it in action. Our brain also undergo a second growth spurt after the one that ended at the age of six, and with the onset of puberty our brain start to "rewire" and restructure into the organ that will become host to our adult mind.
The secondary spurt of neural growth that occur at the age of our sexual awakening also adapts our hormonal and sensory apparatus to our sexual choice, fine-tune the control of our voluntary muscle system to adjust for the additional power and mass, and make the final neural path adjustments in response to our environmental experience and exposure. It is only after this fine-tuning has been completed that our brain can be considered as fully developed, finishing a process that started before our birth.
At least, that is what our current level of knowledge would tell us. As for tomorrow, who knows? We already know that science is investigating the possibility of artificially increasing our neurons in an attempt to treat age related dementia, and there are various other drugs under investigation that promise to boost our mental capacity. Right now I believe in the age old wisdom that you lose it if you don't use it. I never could adapt to using it's modern counterpart: "You snooze, you lose..."