A 72-year-old woman presents with gradual bilateral hearing loss affecting high frequencies more than low frequencies over 5 years. She denies vertigo, tinnitus, or ear discharge. Otoscopy is normal. Audiometry shows bone conduction thresholds of 40 dB HL at 4 kHz, 45 dB HL at 8 kHz, and air conduction thresholds of 42 dB HL at 4 kHz and 48 dB HL at 8 kHz. Speech discrimination is 72%. What is the most likely diagnosis?