Purpose: To compare the bond strengths of young and old dentin using a single-bottle etch-and-rinse adhesive system with two acid-etching times (15 s and 30 s), and to examine the correlation between dentin hardness and bond strength. Materials and Methods: Twenty-four molars from 18- to 22- or 55- to 60-year-old patients were prepared to expose the occlusal dentin, cut into two equal parts, and assigned to four groups (G), varying the etching time (15 s and 30 s) and dentin age (young [Y]or old [O]): G15Y, G30Y, G15O and G30O. After etching, AdperSingleBond (3M ESPE) adhesive was applied and the tooth was built up with a composite (Filtek Z250). The specimens were prepared for the microtensile bond strength test (µTBS) at a crosshead speed of 0.5 mm/min and the interfaces were observed under SEM. Also, the Knoop hardness (KHN) of young vs old dentin was measured. Results: The mean (± SD) µTBS (MPa) were: G15Y = 45.9 (± 10.7)a, G15O = 34.1 (± 9.4)b, G30Y = 48.6 (± 14.3)a and G30O = 47.7 (± 11.0)a. Two-way ANOVA showed no difference in µTBS between young and old dentin. Old dentin acid etched for 15 s (G15O) had a lower µTBS than when acid etched for 30 s (G30O). Dentin hardness was higher for old dentin (67.0 ± 4.8KHN) than young dentin (63.7 ± 2.9KHN) (p 0.004). No correlation between µTBS and dentin hardness was observed. Resin tags were larger and more numerous for young dentin. The hybrid layer formed in intertubular old dentin (G15O) was very thin. Conclusion: Bonding to old dentin with 30 s of etching time resulted in higher bond strength and more homogeneous hybrid layer formation than dentin acid etched for 15 s. Keywords: acid etching, dentin bonding agents, old dentin, bond strength, scanning electron microscopy, microtensile bond strength, microhardness, bonding systems, dentin