Learning biology, chemistry, physics isn't something that happens instantly as a result of development. Rather, they require study and practice.
Yeah, but those subjects (IMHO) are a lot like ballet. There's no reason not to start young, if you have a kid who's so inclined. But you can pick them up for the first time at 12 or so, having had no particular prior experience, and still go on to do them professionally, with sufficient motivation and natural talent.
I do think there's a "too late" point for a self-directed learner to decide on some paths - but that's true of traditionally schooled kids, too. And I definitely agree that, at some point, it takes hard work over a period of time to be really good, at the adult level, in just about anything. Which is why you need sufficient motivation if you start on a path later than your peers and still want to be successful at high levels.
My real-life knowledge of someone who was unschooled was a 15yo who couldn't do 4th grade math. She liked to read, so could read fiction relatively well, but didn't have the skills to read and interpret scientific writing, or to do the sort of literary analysis expected of a kid her age. But I suspect that "unschooling" was a more socially-acceptable explanation than "We got mad at the school district so pulled her out, and then having her around during the day to watch the younger siblings was a good deal for us, and then she was so far behind that she didn't want to be a 13yo in 3rd grade in public school, so we just let her do her own thing."
Which is a far cry from "My kid is currently disinterested in math, so we aren't doing anything formal in math. But there are workbooks on a variety of topics on her bookshelf, a handful of websites that include math games on the home page of her browser, I give her a budget and shopping list at the grocery store and if she can find an extra treat that fits into the budget we'll buy it, and we play Yahtzee as a family every weekend with each player keeping their own score."
