I would say as a rough guesstimate that intelligence as such (a vague concept admittedly) really has fully developed or close to fully developed in the early teen years. Knowledge keeps building.
But “smart”, which is the term Isaac used, colloquially is not limited to intelligence. For example the term “street smarts” refers entirely or almost entirely to knowledge, even to a kind of maturity, gained through a certain kind of experience.
Fluid and Crystallized intelligence (admittedly measured on IQ tests, which are not perfect to say the least) were both found to peak at age 26 by this study (for those who don’t know the distinction between the two types of intelligence, wikipedia explains it rather well). Fluid intelligence levels off between 16 and 18, increases slightly until the mid-twenties, then starts a slow, steady decline. Crystallized intelligence is similar, except it levels off in the early twenties, and decreases much more slowly, though the decrease still starts in the mid-twenties.
Interestingly, the intelligence of people on the lower bound levelled off earlier (by about two years) than that of those on the upper bound.
I would say as a rough guesstimate that intelligence as such (a vague concept admittedly) really has fully developed or close to fully developed in the early teen years. Knowledge keeps building.
But “smart”, which is the term Isaac used, colloquially is not limited to intelligence. For example the term “street smarts” refers entirely or almost entirely to knowledge, even to a kind of maturity, gained through a certain kind of experience.
Fluid and Crystallized intelligence (admittedly measured on IQ tests, which are not perfect to say the least) were both found to peak at age 26 by this study (for those who don’t know the distinction between the two types of intelligence, wikipedia explains it rather well). Fluid intelligence levels off between 16 and 18, increases slightly until the mid-twenties, then starts a slow, steady decline. Crystallized intelligence is similar, except it levels off in the early twenties, and decreases much more slowly, though the decrease still starts in the mid-twenties.
Interestingly, the intelligence of people on the lower bound levelled off earlier (by about two years) than that of those on the upper bound.