My dad, 82 now, has the same issue. But, he's had several cancers and chemo, radiation, etc. The docs told him so much chemo is the cause, as it kills cells (not sure if that's the right term) in the bone. He is too old and/or unhealthy to go through a hip replacement; which is the only 'cure'. He's taking pain pills.
Funny story, my mom is 79 and is very healthy. Still walks and goes to a class 4x a week and all. She rarely takes a 'drug' and instead focuses on nutrition and herbs. She told my sister when they prescribed his pills that she was recluctant to get it filled as she didn't want my dad "hooked on pain pills". They actually tried managing the pain for a few weeks without pain pills.
She relented and now he takes 1/2 at night and 1/2 in the mid morning.
It was the same with me growing up. Until I was 40 I would still only take a half a tylenol at most, and it all goes back to my mom and the way she's always been about that stuff.