ONAGAWA, Japan — At age 39, Yoshiaki Suda, the new mayor of this town that was destroyed by last March’s tsunami, oversees a community where the votes, money and influence lie among its large population of graying residents. But for Onagawa to have a future, he must rebuild it in such a way as to make it attractive to those of his generation and younger.

“That’s the most difficult problem,” Mr. Suda said. “For whom are we rebuilding?”

The reconstruction of Onagawa and the rest of the coast where the tsunami hit is a preview of what may be the most critical test Japan will face in the decades ahead. In a country where power rests disproportionately among older people, how does Japan, which has the world’s most rapidly aging population, use its dwindling resources to build a society that looks to the future as much as to the past?

The clashing generational interests are perhaps most striking here in Onagawa, a town of 8,500 residents whose average age of 49.5 is above the national average of 45. The evolving debate over the shape of Onagawa’s reconstruction underscores how older Japanese, more attached to their land and customs, are wielding disproportionate influence and swaying local governments into issuing reconstruction blueprints at odds with Tokyo’s stated goal of creating long-term sustainable communities.

The debate here centers on the future of Onagawa’s rapidly aging and depopulated fishing villages, which, reachable only by twisting mountain roads, dot peninsulas that spread east and south of the town center here. Three other villages, located on two nearby islands, depend on a ferry that runs only three days a week for access to the mainland.