Slow Starters Have Higher IQ?
I suspect that this is unlikely, and not because Nature is the answer to everything, but because I don't think that being made fun of for being behind makes 100% of those students work harder to make themselves smarter.
The real finding here provides dramatic support to the Nature side of the debate. Students that end up being identified as the most intelligent are those whose cortices (the site of higher cognitive thought) continue to develop for longer, hitting their peak much later than their less gifted counterparts. As such, two possible explanations jump to mind for why students whose cortical growth peaks latest are those which are the smartest: 1) the cortex simply develops for longer, or 2) these kids have had a much richer set of experiences by the time their cortices stop growing. In either case, the growth of the cortex is probably heavily determined by genetics, but presumably can be affected by Nurture to a certain extent.
Although I don't remember the details now, there was a study done somewhere in Europe about a decade ago on a sperm bank that accepted donations only from those men judged highly intelligent to try to figure out how much of the result childrens' intelligence was attributable to the biological father, and how much was due to the conditions in which each child was raised. These kids ended up having an unusually large number of prodigies, which suggests to me that Nature (for better or worse) plays the major role in determining intelligence. That being said, if you're a horrible parent, you'll screw life up for your kid, and he/she may never be able to realize his/her potential.