There are some really fascinating concepts concerning the brain, it's ability to learn, remember, forget and so on. Most athletes are familiar with "muscle memory", as would be most musicians who have taken the time to master an instrument. I read a great paper on this theory that our consciousness is made up of interconnected and overlapping neural nets. Each net represents a sphere of the physical world around us. For example, you have a layer of your brain dedicated to the space around your physical person. It knows where the mouse is when your hand isn't upon it, knows where the floor ought to be when you take that next step, etc. There's also the layer of our immediate senses. Then there's the layer of short term, still being processed, memory. I think our ability to learn depends on the thickness of this region and the strength of it's connections to our long term memory and senses determine how quickly we learn and how well we remember. Most memory can be indexed by the brain with sensory input or associative imagery. So as we learn new things, our brain is amazingly abstracting them and connecting them to familiar concepts. That's why it's easy to pick up a new language once you've trained your brain to learn new languages. Learning that first one though falls on a concept known as neural plasticity.
Our brains are amazing in that we can rewire them almost at will by forcing ourselves to learn new things. I had a great experience with this 6 years ago when I started getting symptoms of carpal tunnel syndrome in my left wrist. Turns out the Qwerty key map is very left hand intensive. A little research led me to the DVORAK key map, which was designed to balance the keyboard use between left and right hands for the English language. It took me 3 months of every day use to get proficient and another 3 to get my speed back up to where I was with Qwerty. Now I am past the speed of Qwerty as the layout is 20% more efficient.
So I had to rewire my brain, force feeding it new muscle memory, to get where I wanted to go. I believe, because of this experience, there is nothing we can not learn with sufficient persistence. It took me 8 hours on a Saturday to become proficient at throwing mud on a course of brick with a trowel. It took my daughter about 20 - 30 minutes to learn how to flip a pan cake with a spatula "just right". But in both cases it will take us many days of repetition to really solidify those pathways in our brains and achieve any level of mastery.
Taking this all into account, I have come to two conclusions. You can learn as much as you want and the more you push your brain to learn new things, the more nimble it is. So I think it's particularly important, as we age, to keep learning new things - programming languages included.