A 72-year-old woman presents with gradual bilateral hearing loss over 10 years and difficulty understanding speech in noisy environments. She denies tinnitus or vertigo. Otoscopy is normal. Audiometry shows air conduction thresholds of 20 dB HL at 500 Hz, 25 dB HL at 1000 Hz, 35 dB HL at 2000 Hz, 50 dB HL at 4000 Hz, and 55 dB HL at 8000 Hz. Bone conduction thresholds are elevated equally across all frequencies (within 5 dB of air conduction). Speech discrimination is 60%. What is the most likely diagnosis?