Another things is that we don't always seek out the things we are interested in on our own because we are ignorant of them and it's not until we've studied them

Another things is that we don't always seek out the things we are interested in on our own because we are ignorant of them and it's not until we've studied them we realize that we enjoy the subject (I find this is very true for math). The way I see it is that we learn a broad range of subject through our life's but each choose to specialize in what we find the most interesting. The further we get with our education the more we can narrow down what it is we find interesting (or maybe we just exclude the things that aren't). Even if we simply flush our systems completely (which I don't believe) it's much easier to re-learn something you've once learned than to learn it for the first time. It's not lost it's just not readily available.