Intelligence is complex and malliable. The brain is like a muscle and can be made stronger by learning and experience. Conversely if you don't use your brain you lose it. Sometimes people's IQ does change during teens.
www.bbc.co.uk/news/science-environment-15369851
I feel that secondary schools that stream children by CATS results do potential damage to the children's self esteem and waste potential. Surely it would be better to have mixed ablity for the first term and then put children into sets when they have settled into secondary school. Or prehaps group children according to their primary school teacher's recommendation.
One of the secondary schools near me streams (yes, steam not set for individual subjects) on the basis of a 90 minute CATS test taken in September. I can see the benefit in CATS tests to identify special needs, but I feel its far too crude a tool for assessing ablity.