I'd say anywhere from 100 to 130 years old. Old Empire.
In the 1700's a full calf binding would have been generally different to your image. The spine bound very differently. Quarto more common back then, whereas octavo is the thing nowadays.
Later, the leaves were folded into groups and stuck, with the now more common cloth spines being detached / separate from the contents.
Incidentally, most of Byrons first editions had soft covers. Poor quality paper stock, too.
