I'm not sure that your experience of feeling smarter contradicts cperciva's point about intelligence being determined early in life. I can see that there could be a difference between intelligence as some kind of innate capacity and how that capacity is applied to various tasks.
You can certainly learn more stuff and so be able to do more difficult things, but your ability to learn and apply knowledge may not be changing.
This is a good point. I agree that my feeling smarter does not actually mean that I am smarter. I -do- produce a lot more, but this could be due to other factors.
I think you're hitting very close to the important issue here which is that all of this is very difficult to define/think about without a really good idea of what sorts of computations our brains perform.
My point isn't really so much that I think it can change, even, it's that we should be way more skeptical when it comes to making claims about subtle concepts like these. Usually I find that those who say "we are only really growing intellectually until we're 5" have a very limited and kind of rigid conception of intelligence.
You can certainly learn more stuff and so be able to do more difficult things, but your ability to learn and apply knowledge may not be changing.