Adam Stephens is a warlock, taught to be one by Endora, much to Samantha and Darrin's dismay. His parents are Samantha and Darrin, and his sister is Tabitha. Adam was born in 1969. In Tabitha, he is a mortal like Darrin instead of a warlock.
In the episode "Adam, Warlock or Washout" it was discovered that Adam had supernatural powers. Apparently Adams's powers were latent very deep inside him. At first Adam was thought to be a normal mortal human. He tested out as 'mortal' during a test, in which the same test had proved that Tabitha was a witch. Adam's grandfather, Maurice, cast some sort of weird spell upon Adam by using a shell. The 'shell' spell was very confusing, but it ended up releasing Adam's latent powers that were there the entire time. Adam then immediately began his "wishcraft"...just as Tabitha had done when her powers first woke.
During the late 70's a "Tabitha" spinoff was created. In the first Tabitha pilot, Tabitha was going to fall for a mortal man...just as her mother, Samantha, had done. But instead of a meddling witch of a mother-in-law....Tabitha would have a warlock meddling brother. Adam would pop in and out, trying to convince Tabitha to leave mortals alone and to pursue her life as a witch. This pilot was bought. But before it aired the entire premise was changed. Adam became mortal...trying to convince his witch sister, Tabitha, to give up her witchly ways. A new aunt was also added...who would dislike Adam, and try to get Tabitha to act more like a witch.