Jeb Bush set out an aggressive economic growth target in his campaign announcement speech Monday: four percent real G.D.P. growth, for a decade.

“It’s possible,” he said. “It can be done.”

Don’t bet on it.

Over the last 40 years, the American economy has grown at an average of 2.8 percent per year. That’s slower than the 3.7 percent average from 1948 to 1975, but the future looks even gloomier because that 2.8 figure relied on two favorable trends that are now over: women entering the work force, and baby boomers reaching their prime earning years.

After 2020, with the percentage of the American population that is of prime working age shrinking, the Congressional Budget Office expects growth to stabilize at 2.2 percent. Hitting Mr. Bush’s target would require nearly doubling that pace. It would mean exceeding the economic performance of every presidential administration since the Kennedy-Johnson years despite demographic headwinds caused by baby-boom retirements.

“My personal goal is to be 6 feet tall and have a six-pack stomach, but I don’t think most people would think either Governor Bush or myself have a very viable plan for reaching our targets,” said Gene Sperling, who led the National Economic Council under Presidents Clinton and Obama. He is 5 feet 5 inches tall, and The Times has not reviewed his abs.