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Getting old isn't cheap. Especially in San Antonio.Retirement-age folks in the Alamo City lead the nation in non-mortgage debt, according to a new study by Lending Tree , the online loan marketplace.Excluding mortgages, San Antonio residents between 65 and 70 years of age carried a median debt balance of $29,993 — or $7,659 more than those in Memphis, Tenn., the metro with the lowest debt balance.Apparently, other Texas seniors are also lugging around hefty debts. Austin, Houston and Dallas — in that order — took the next three slots on the list. In all four cities, auto loans made up the biggest share of the obligations, according to the study."My personal theory is that it's trucks; they're expensive," Lending Tree Senior Research Analyst Kalie McFadden explained to CNBC Well, yee-haw.But we're guessing the relatively paltry retirement packages offered up by San Antonio's tourism-driven economy also may factor in.