well into her fifties = before her sixty (she's regular smoker)?

Well into her 50s would probably mean somewhere between the age of 54-59.

That means she smoked until that time. She could be 70 and have stopped smoking at the age of 58.

well into her fifties = before her sixtieth birthday(she's regular smoker)?
Yes almost exactly. Infers age 54/55? or more

Thank you both.

I would put it at 52 or 53. I would personally use "well into" an age decade if I meant that the person was appreciably into that decade, but not necessarily the later part. I would equate "well into" the fifties with "older than fifty but not yet into the middle or late fifties".