When developmentalists looked at the two kinds of intelligence, fluid and crystallized, to determine if intelligence continues to grow, slow, or decline with age, they determined that?

Research from developmental psychology suggests that crystallized intelligence grows with age, while fluid intelligence declines with age.

1. Fluid intelligence- Fluid intelligence refers to the ability to perform tasks that require speed, acuity, problem-solving skills, alertness, abstract reasoning and learning new tasks. Fluid intelligence peaks during a person's 20s, but  declines during late adulthood due to natural brain degeneration that occurs from ageing. 

2. Crystallized intelligence- Crystallized intelligence refers to the knowledge, information and "wisdom" a person has accumulated through their lifespan. Crystallized intelligence increases with age, since a person who has lived longer has had a longer time period to learn, grow their knowledge and experience various phenomena.