The so-called fluid intelligence is a cognitive ability based on physiology. It is the ability to recognize, remember and understand things, such as perception, memory, operation speed and reasoning ability, which will decline with age, and its development is closely related to age. Generally, fluid intelligence reaches its peak after the age of 20, and will decline with the increase of age after the age of 30, and is less influenced by education and culture.
It depends on innate endowment and improves with the maturity of nervous system. Its characteristic is that it can judge unfamiliar things quickly and accurately.
The other is crystal intelligence, which is cognitive ability based on empirical knowledge, such as learned skills, language ability, judgment, association and so on. This is related to education and culture, but not to age. Crystal intelligence does not decrease with age, but gradually develops slowly after the age of 25.