Final answer:
The oldest-old primarily invest in the domain of meaning of life, as they shift focus from career and earnings to seeking fulfillment and psychosocial development.
Step-by-step explanation:
The domain the oldest-old invest in most is meaning of life. As individuals reach the stage of being 'oldest-old,' they have often passed the phase of career and earnings, which peaks during middle adulthood. The focus shifts from being defined by careers to seeking meaning through various avenues such as psychosocial development, life experiences, and social connectedness. This is supported by theories which suggest that finding meaning is crucial throughout our lives, particularly as we age.
Despite the potential cognitive decline, maintaining cognitive fitness through activities such as mental exercises, social interaction, and even adopting certain diets can help delay the onset of cognitive impairments. Functionalists propose that those with better resources who remain socially and mentally active have a better adjustment in old age. Therefore, investing in cognitive activities and health can contribute towards a meaningful life in old age, but the primary pursuit often becomes the search for deeper meaning and fulfillment.