Teenage Caveman (2002) on Netflix

Plot summary

A viral epidemic has killed most of humanity, and the survivors have gone back to a primitive tribal life in caves. David is the son of the tribe's leader, an abusive man. When his father sexually assaults Sarah, David's girlfriend, David kills him, and he is banished along with his friends. The group leaves the caves and comes upon a ruined city. In the middle of a storm, two locals, Elizabeth and Neil, rescue them. The pair are the only two inhabitants of a solar-powered city, and they draw the teenagers into sex and drugs, vices that their tribe forbids.