Aging is associated with changes in frontal, striatal, and medial temporal structure and functioning. These brain regions play a central role in deciding between choices of different values. In a series of experiments involving lottery choice decisions, we evaluated young and older adult differences in risk-taking behavior and associated neural processing in these brain regions. In the lottery choice task, decisions to follow expected values as objectively presented are more optimal than biases to apply subjective valuation heuristics. Our findings revealed that older adults are more risk-taking than younger adults in lottery choice decisions, particularly for stakes associated with likely losses. Critically, younger but not older adult ventral striatum showed sensitivity to increasing stake probabilities. Instead, older adults engaged medial temporal and ventral medial areas to increasing stake probabilities, and dorsolateral and dorsomedial frontal areas to decreasing stake probabilities. Older adults also displayed substantial individual differences such that those with greater neural sensitivity to expected value in medial temporal, striatal, and medial frontal areas showed more extreme risk-preferences. Finally, modulating emotional feedback affected older adult decisions more than younger adults, suggesting the greater reliance on affective processing for value-based decisions with age. Our findings are consistent with a shift in the neural computation of value-based decisions from young to older adults that stems from age-related differences in the subjective value of probabilistic events. We consider these findings in the light of a new theory on the effects of age on self-organizing systems interacting with dynamic environments.