I haven't read the whole article, but fwiw, DS7 (just!) is clearly more than 2 years advanced through Piaget's stages; he (unsurprisingly to me) passes the first handful of tests I can find for being in the formal operational stage, which Piaget reckoned started around age 11, and others have argued begins later or for some people never. Specifically he easily passed the two-headed green parrot problem I mentioned before, and this one:
http://tlccvc.org/piaget2.htmThe EK47 problem here
http://webspace.ship.edu/cgboer/piaget.html took him a couple of minutes, and listening to him thinking it through without saying anything myself was painful, but he got there in the end! IIRR, this one is much-researched and many adults, including university students, still fail.
In his case, the reason is clear: he got bitten by the formal logic bug more than a year ago and has had lots of practice at thinking in formal logical terms. Most 7yos have simply not had that exposure. (And I dare say many would not have wanted it - but I just don't buy this as being a non-cultural cognitive development issue.)
