Writer Simon Kinberg explains the post-credit scene for X-Men: Days Of Future Past.
"The post-credit sequence is a tease into what will be the Apocalypse movie” he explained. “Now the Apocalypse you see – that young boy – may not be the actor who plays the character in our movie. It’s a deep back story glimpse into the character, but our movie doesn’t take place in ancient Egypt and will likely not star a young boy Apocalypse.”
In the Marvel Comics, Apocalypse was born in Ancient Egypt and found as a grey-skinned infant given the name En Sabah Nur, which means “The First One.”