The mental ability of teenagers can improve or decline on a far greater scale than previously thought, according to new research.Until now the assumption has been that intellectual capacity, as measured by IQ, stays quite static during life, BBC reported.But tests conducted on teenagers at an average age of 14 and then repeated when their average age was nearly 18 found improvements - and deterioration.The findings are published in the journal Nature.They have implications for how pupils are assessed, and the age at which decisions about their futures are made.This study involved 19 boys and 14 girls, all undergoing a combination of brain scans and verbal and non-verbal IQ tests in 2004 and then in 2008.The results show that a change in verbal IQ was found in 39% of the teenagers, with 21% showing a change in \"performance IQ\" - a test of spatial reasoning.