An era is ending in the advertising industry as Robert J. Coen, who has been predicting and following trends for advertising spending through 12 U.S. presidencies, steps down from his post as the lead forecaster at a unit of the Interpublic Group of Companies.

Mr. Coen, who is 86, has spent six decades at units of Interpublic. His career actually predates Interpublic; when he began at McCann-Erickson in New York in 1948, Interpublic was 13 years from being incorporated.

‘‘Bob Coen is the unequaled dean of advertising forecasting,’’ said Leland Westerfield, managing director atBMO Capital Markets, ‘‘and the benchmark by which all those in advertising, media and finance have gauged their own expectations for the advertising economy.’’ Most recently, Mr. Coen has been senior vice president and forecasting director at a media agency named Magna, part of the Mediabrands division of Interpublic.

Mr. Coen will become an adviser to Magna, Mediabrands is to announce Monday, as he passes his lead forecasting role to BrianWieser, a senior vice president at Magna who was being groomed for a larger role at the agency.

Mr. Wieser, 34, will become senior vice president and global forecasting director at Magna; the post Mr. Coen has held since Harry S. Truman was in the White House is thus expanded to take on more of a worldwide purview.

‘‘It’s a good way to pass the torch along,’’ Mr. Westerfield said.

When Mr. Coen started 61 years ago, advertising spending in the United States totaled — according to his own data, of course — almost $4.9 billion.

By comparison, his most recent forecast for 2009 calls for ad spending to reach $258.7 billion.

Stepping down ‘‘was something I thought about periodically in the last couple of years,’’ Mr. Coen said in an interview. ‘‘I knew it would eventually come.’’ As for the choice of Mr. Wieser to succeed him, Mr. Coen said, demonstrating his characteristically dry humor: ‘‘Brian’s been around here for a while. It’s about time he started doing some of the work, the real work.’’