This paper examines how traditional informal institutions shape preferences for old-age support and the emergence of initiative-based long-term financial planning. Using data from the China Health and Retirement Longitudinal Study combined with clan genealogy records, we estimate the causal impact of exposure to clan culture on old-age support preferences. We find that stronger exposure to clan culture in early life is associated with a higher likelihood of adopting modern preferences for old-age support, characterized by greater reliance on self-initiated savings and pension participation rather than dependence on children. The results are robust to instrumental variable estimation and placebo test. Importantly, the effect operates exclusively through exposure to clan culture during early life, while clan culture encountered in adulthood has no significant influence, particularly among individuals who relocate later in life. Mechanism analysis suggests that peer effects, generalized trust, income growth, and financial literacy jointly mediate the relationship between clan culture and old-age support preferences. We further show that the expansion of the formal pension system complements, rather than crowds out, the influence of clan culture, consistent with a process of co-evolution between informal and formal institutions. The effects are stronger among individuals with favorable attitudes toward clans, rural residents, and men. Finally, modern preferences induced by clan culture translate into higher pension participation and larger financial asset holdings. Overall, the findings highlight how informal institutions can shape economic preferences and facilitate the adoption of formal financial arrangements.