Final answer:
Numeric ability does not start to decline around age 25 according to Schaie's research; other cognitive functions like fluid intelligence do slow down in late adulthood.
Step-by-step explanation:
According to K. Warner Schaie's research on cognitive development and decline, the abilities that start a gradual decline at about age 25 are mechanical reasoning, spatial orientation, and inductive reasoning. Numeric ability, on the other hand, does not start to decline around this age. Schaie's studies reveal that certain aspects of cognitive function such as fluid intelligence—which involves information processing, reasoning, and memory—begin to slow down in late adulthood. However, crystallized intelligence, which includes the accumulation of knowledge, skills, and strategies through lifetime experience, tends to remain stable or even improve as we age.
Based on the provided information, mechanical reasoning is the ability that does not start to decline at about age 25. The cognitive abilities that tend to decline with age are fluid intelligence, which includes information processing abilities, reasoning, and memory.