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.