With an increased interest in nongame fishes such as buffalofishes (Catostomidae, Ictiobus), there is a need for better foundational data on their life history. Bigmouth Buffalo I. cyprinellus, for example, have been found to live for more than a century. Age estimates for other sucker species have similarly suggested long life spans, but validation studies as reference points are often lacking. We conducted a 3-year study on Smallmouth Buffalo I. bubalus in Oklahoma to validate annual increments on three hard part structures (otoliths [lapilli], pectoral fin rays, and opercula) typically used for age estimation. We marked wild fish with oxytetracycline (OTC) injection and stocked those fish into a hatchery pond to create a population of fish with known times since marking. Furthermore, reproduction in the pond allowed us to validate annulus formation in young fish. We analyzed 117 fish and found that otoliths were more reliable, precise, and accurate than the other two structures for detecting OTC marks and counting annuli. Age estimates, from 1 to 61 years, were greatest when otoliths were used, with 99% of estimates corresponding to known time since marking or known age. Otoliths appear to be the only reliable structure for accurately estimating the age of Smallmouth Buffalo within 1 year of actual age, and their use indicates that this species can live for more than six decades in Oklahoma.