Some people believe that IQ decreases after 20 years of age while some say that it increases after 20. Who should we trust? In fact, both of the scenarios are right.
Experts say that childhood is the best time to learn new things and increase intelligence. This is because around this time, the mind is an empty canvas waiting to be filled. So, the way a child is brought up, introduced to knowledge, studies and environment plays a vital role in building their intelligence.
It should be remembered that IQ is not a single measure, it is a collection of different factors. A lot of traits add up to produce an IQ score of a person. Now, as a person ages, some of these traits or abilities increase while some decreases.
As per a study published by Ian Deary in 2009 (Age-associated cognitive decline ), you can change your verbal and numerical abilities after you hit 20 but up to the age of 50. On the other hand, perceptual speed, spatial orientation and inductive reasoning tend to decline when a person reaches 25 years of age. From this study, it can be concluded that you can change your IQ by gaining more knowledge and experience, but age plays a very important role in this regard.
Other than this, environment is another factor that can change people's IQ. If we go back into the past, the average IQ of a 20 year old was less compared to the 20 year old of 2002.
Between these years, there was a 18 point increase in the average IQ of a person. This shows just how the environment can impact a person's score. There were many new inventions and upgrades introduced in the last few decades and they all helped give people's intelligence a boost. But, again, opinion on this is divided as many people that dependence on technology causes people to become less intelligent.
Various studies say that school also plays a vital role in changing a person's IQ. This is done by teaching the value of taxonomization to kids which is basically to group things in a systematic way and find better ways of understanding things with their own mind. Flynn effect can be studied to understand this better.
For example, some studies showed how the intelligence of a taxi driver changes with time, from when they got into the business to being in the business for a few years.
They memorize the maps and make use of their navigational trait a lot more, hence the change. Similarly, the brain of a juggler is also said to show changes due to making efforts at improving focus and concentration.
Looking into the future, the average IQ of people is to rise again because the environment is yet again changing and we already have examples before us in the form of technology, smart minds etc.
So, yes. The IQ can change over time but it requires the person to change their environment, put in extra effort and go beyond the horizon of what they're capable of doing.