The research presented at the American Heart Association annual meeting analyzed 50 different studies on running fitness collected from 28 countries.
The data included 25 million youths and spanned 1964 to 2010.
Researchers said the findings are the first evidence that "cardiovascular fitness has declined around the globe since about 1975."
Across the board, endurance fell by about five percent per decade, leaving kids around 15 percent less fit in cardiovascular terms than their parents were.
Clocking a one-mile (1.6 kilometer) run, today's kids are a minute and a half slower than their counterparts 30 years ago.
"If a young person is generally unfit now, then they are more likely to develop conditions like heart disease later in life," said lead author Grant Tomkinson, senior lecturer in the University of South Australia's School of Health Sciences.
"The most important type of fitness for good health is cardiovascular fitness, which is the ability to exercise vigorously for a long time, like running multiple laps around an oval track."
The declines in cardiovascular endurance performance went hand in hand with individual countries' measurements of average body fat and obesity.
"About 30 percent to 60 percent of the declines in endurance running performance can be explained by increases in fat mass," Tomkinson said.
Kids are urged to do an hour of exercise daily in order to maintain physical fitness.