I stumbled upon a blog recently and added it to my blogroll, because it was the quickest way to find it again. I am not sure of the writer's age, but judging by his picture found elsewhere on his website, "young thoughtful adult" is the appendage I would apply if pressed.
The April 6 entry contains some interesting philosophical musings and the rest of this post is really in reply to that.
I believe you are right in that there is nothing we do or think that does not contain all the elements of where we have been, and all the things that make up who we might be in this moment. But I am not so sure that ideas exist without the thinker or thinkers. That thought and your others have shades of Plato's ideal forms, which in itself isn't entirely flawed, but without life and a thinker to think them - I believe that ideas and forms are naught.
Though the thought has occurred to me before, I don't believe that everything to be known already exists. Instead I think that each new generation of humans combines, adds to and expands the knowledge base that everyone else uses. We leapfrog on the backs of those who came before.
People cannot separate themselves from themselves, but only attempt to apply the clearest, logical thought processes to various "ideas", concepts and theories (I can't help it, Star Trek Vulcans come to mind). The more we know and understand ourselves and our 'programming', the more we can know how to apply the least amount of junk from our particular journey. But ideas will never be devoid our individual particular twists. Each thinker understands things from his/her own unique perspective.
Of course, in thinking on the ideas of others and following our own individually flavored process, we forever change who we are. And in like fashion, it is those unique twists of individual thought processes (made possible by our individual journeys) that change ideas and take knowledge to completely new places.
More on Platonic Forms found in a quick search.