David Perel spends at least an hour a day on his iPhone 6. He watches live sports on television and tries to catch “The Late Show With Stephen Colbert” on CBS, but he rarely tunes in to prime-time shows. He does not have a Netflix subscription, but he is thinking hard about getting one.

Mr. Perel, 26, is typical of a generation that grew up on the Internet. These young adults watch less traditional television and do not have a reputation for reading print newspapers or magazines. They skip online commercials and block ads on their web browsers and phones.

No wonder marketers feel as if they are starting over.

“Everything you know about advertising — chuck it,” said Laura Desmond, chief executive of the media agency the Starcom MediaVest Group. “It’s a completely different world and game with millennials.”