Mortals are living creatures that are able to age and die. Demigods are half-human, thus they are alternatively a mortal as well.
The first humans were created by the Titan Prometheus. They existed before the Olympians rose to power, existing under the rule of the Titans. They existed in three other times before finally existing with the gods; the Golden age that was peaceful and, the Silver age and the Bronze age. Then the Iron age, but with Zeus still awaiting his revenge on Prometheus for giving humans fire, gave the first woman of the Iron age, Pandora, insatiable curiosity. With nothing stopping Pandora, she opened the Pithous given to her by Zeus.
While demigods were the children of gods and a human parent, they were also mortal as demigods didn't live forever and still died. Many demigods attained a manner of immortality during their lives or after death. The hero Heracles attained immortality and godhood after death, and Dionysus attained godhood during his lifetime.
Almost all known demigods had families, most of which maintained one or several different family lines, such as Perseus and Heracles.