The move was approved in October and took effect Friday.
All U.S. beef was banned from Japan in 2003, following an outbreak of mad cow disease in the American herd. The ban was lifted in 2005, but only meat from cattle less than 20 months old was permitted to be imported, the Japan Times said.
The commission noted the difference in risk between 20-month-old and 30-month-old cattle was "extremely small."
Exports generally involve meat from cattle between 16 and 22 months old, Japan's Agriculture, Forestry and Fisheries Ministry noted.
The easing of the rules means about 90 percent of all U.S.-produced beef will qualify for export to Japan, the newspaper said.