alt Hacker NewsIQ is highly heritable, so I don't think there is any environmental factor that has this big an impact, except in extreme cases like severe malnutrition. Also note that IQ increases with age roughly into early adulthood independently of environment, so the IQ of a 14 year old increasing is a perfectly normal part of the heritable parts of development.
What's "highly"? It's likely somewhere between 15%-50% --- weak-to-medium, if you read it as a correlation coefficient.