Teens aren't too old to boost their IQ, study finds

I haven't looked at this study closely but it seems to represent  confirmation of the most favoured explanation for the Flynn effect.  The Flynn effect is that average IQ scores rose during most of the 20th century in most places.  The effect is probably the result of a number of influences but the influence generally thought most important is  increasing test sophistication.  The longer kids stay in  school the more they become "test wise" and thus increase their IQ score even though there is no increase in underlying ability.  That would certainly explain the findings below but, sadly for the hopeful, no real advantage will have been gained

 If your teenager could use a few more IQ points, Norwegian scientists have some good news: It may not be too late for junior to get them.

Many researchers now agree that mental stimulation in one's early years helps IQ to develop, but there is no such consensus that education - or anything else - can boost IQ on older kids. Studies have seen correlations between a person's total years of schooling and his or her IQ, but there's no good way to tease out the cause and effect. It could be that extra school raises IQ, but it's just as likely that those with higher IQs to start with are inclined to stay in school longer. It's also possible that some other trait, such as family income, influences both IQ and length of education at the same time.

In an ideal world, researchers would divide students into groups, give some of those groups a few extra years in the classroom and then measure everyone's IQ. If additional education was indeed an intelligence booster, then the students who spent more time in school would have higher IQs, on average, than the students who spent less time in school.

It turns out that the government of Norway conducted just such an experiment - albeit unwittingly. From 1955 to 1972, the Norwegian government required schools to increase the number of years of mandatory schooling from seven to nine. This meant that students who used to be done at age 14 now remained in the classroom until age 16. School districts didn't implement the change all at once but rolled it out over many years. This resulted in a data set that allowed researchers to slice and dice the figures in many ways - to check their work, in other words.

The other helpful thing about Norway is that the military there measured the IQ of all 19-year-old men as part of the universal draft.

Researchers from the University of Oslo and Statistics Norway (the government's bureau of statistics) matched up IQ and years of schooling and IQ for men born in 1950 through 1958. They found that each of the additional years of education raised the men's IQ by an average of 3.7 points - an increase that was deemed statistically significant. For these men, the school reform meant that they got about two additional months of education, resulting in an additional 0.6 IQ points.

The results were reported in Proceedings of the National Academy of Sciences.

"These results do not directly challenge the recent emphasis placed on early childhood environment for the development of cognitive skills," the researchers wrote. However, they added, "these results suggest that we should not yet entirely disregard the potential of interventions even as late as in adolescence."

SOURCE