Answered on Quora.
Yes, oddly enough, IQ does move around somewhat, including gains or losses of up to 10–15 points, during adolescence. It is not quite known why IQ can move around a bit in adolescence, but in that stage of life, you can move around in a lot of ways. Your personality is not fully formed yet, so we cannot diagnose personality disorders in adolescence.
In addition, you can intervene with some dangerous adolescents, and if you work hard enough, you can make some good progress with them. I recall a young man who seemed to be headed for a career as a rapist, but they grabbed him as a teenager and threw him into intensive therapy. He’s now 40–50, and he hasn’t raped anyone yet. Some adolescents may be on track to seriously assault, attempt to kill or kill other people. I believe that if grab them early enough as teenagers and work hard on them, we can at least get to where they don’t kill anyone during their lifetimes. I have had some good success with people like this myself.
In adulthood, your IQ gets a lot more stuck and it’s hard to raise it. Long ago when I was in high school, a friend told me that a psychologist told him that you could raise your IQ ~15 points even in adulthood if you really put a Herculean effort into it, but it’s so hard to do, that most don’t do it.
He also said that you hit a ceiling at 15 IQ points gain, and you can’t gain any more than that. I think you might be able to lose ~15 IQ points if you sit on your butt, never think or open a book, or stay stoned or drunk a lot. But you will probably hit a floor where you can’t drop it anymore no matter how much of a slacker you are.
Large IQ declines are sometimes seen in illnesses, particularly illnesses of the brain. There is a woman on Quora who documented I believe a 57 point drop in IQ due to her Multiple Sclerosis. She was at Genius IQ before and she fell down to Low Normal. Other MS sufferers also complain of IQ drops. MS is a disease of the brain, so it makes sense.
Answered on Quora.