One side of being an adult learner is that this is self-directed. I don't know what comes before what; I want, for example, a good math education. What does a good math education involve? I don't know! I mean, one of the big things college does is that it serializes your learning. Gives it order. I kind of do the same thing when I read; one book will recommend another in an introduction; with the kindle, I'll often stop reading the first book, go buy and read the recommended book, then return (or not) to the first book. But that happens a lot more in Literature than in Mathematics books, it would seem.
I type in 'calculus' and it seems to start with calculus, rather than giving me all the prerequisites. It does have a nice list of links (most of which I've seen before, but that I've decided I need more math background to properly digest)
The prerequisites, in my experience, are super important. if you skip the prerequisites and think you understand, you are probably getting the "Malcom Gladwell" version of the subject; which in many ways is worse than nothing at all; that's how we've got so many people who have even less math knowledge than I do who think they can explain quantum physics to me. I at least know enough to know that I don't understand, I understand that shrodinger's paradox is not enough to understand what's actually going on; I mean, I know the paradox, and explaining it to me slowly saying, "You understand?" won't help. I need the math background to give the paradox meaning. You do too.
I guess the other problem here is that for basic math, Khan academy is super hard to beat. I can tell Khan "I want to learn calculus" and it will make me do all the prerequisites for calculus. I think that doing this for basic math after Khan is a little like starting a new encyclopedia after Wikipedia; I'm not saying that creating value there is impossible, but you have a hell of a competitor.