Probably even quicker than an adult from my point of view. But then again, computers were much simpler in my childhood days. As an aside: It still bugs me that I don't have anymore access to this kind of fearless, joyfull learning today.
What I mean is: If I would pick up Arduino now I'd fail to make rapid progress.
Over the years it became harder and harder for me to just innocently interact with a new domain. I've acquired so many mental frameworks that clutter up my thinking.
If I run into a problem nowadays I'd frequently resort to reading things up (instead of joyful playing around). And this, of course, would only happen after I spend an enormous amount of time finding the best material and so on. Or I'd wonder from time to time whether Arduino actually is something for me and think about how else I could spend my time more usefully.
The reasons may sound silly but they are very real.
Nah I struggle through similar reasons. I think most people do; it's hard not to when there's practically infinite access to information. For me at least, I find it's a matter of just diving in with a slight "f*ck it" attitude - if I later find out I didn't start with the "best" material, or that something doesn't interest me as much as I thought it would, I try to just cut my losses and move on. Personally, I find that "joyful learning sensation" is more a matter of mental perspective than anything else. For what it's worth :)