She was in her late fifties and looked every day of it.
In this instance, what does looked every day of it mean?
She was in her late fifties and looked every day of it.
In this instance, what does looked every day of it mean?
In the example you provided it means that she really looked her age. It's a stylistic way of saying that she absolutely did not look any younger than what she was.
For example:
She was 55 and looked every day of it.
The woman could look 55 or older, but certainly not younger.