These are the essential statistics to know about the military pension system: the average person retires at age 42 at half pay, and the program will cost taxpayers $18.3 billion next year.

Pentagon pensions, long a target of military reformers, came under assault again yesterday as Office of Management and Budget Director David A. Stockman called them "a scandal" and "an outrage."

Despite such criticism, President Reagan's 1986 budget proposes only a modest change -- a one-year delay in cost-of-living increases for the 1.5 million military retirees. This would save $491 million.

A Pentagon commission last year recommended substantial reductions in retirement pay for future military personnel who leave after 20 years of service. Although legislation was promised, top military officials strongly objected to the plan and Defense Secretary Caspar W. Weinberger has made no recommendation about changing the pension system.

"A decision has been made to hold off on the matter," said Maj. Peter Wyro, a Pentagon spokesman. He said Weinberger could not suggest changes in pensions until he knew whether Congress would approve his proposal for a 3 percent boost in military pay this year.

Wyro said the system "has a lot to do with the retention of well-trained and qualified individuals in the military." Unlike a private pension, he said, "We don't regard it as an employment arrangement; it's a complete way of life. It incorporates a different ethic and different values. We look at changes to the military retirement system with an awful lot of scrutiny."

OMB spokesman Edwin L. Dale Jr., however, said, "The Defense Department strongly resists any change in the military pension system . . . . I've often said that the Joint Chiefs of Staff would rather cut two cruisers than one penny from the retirement system."

Rep. Les Aspin (D-Wis.), new chairman of the House Armed Services Committee, warned Pentagon officials last year that if they did not reform the retirement system, Congress would do it for them. An aide to Aspin said yesterday that the Pentagon has "stonewalled" on the pension issue and that Aspin "wants to bring it to a head."

Criticism of military pensions, whose cost has grown more than fivefold since 1970, has focused on the practice of providing pensions after 20 years of service.

"The average military careerist leaves the service at the age of 42.3 years . . . and goes on to a second career and a second income while collecting his military pension," Aspin said in 1981. " . . . I know of officers approaching the 20-year point who mark the remaining days off a calendar while doing as little work as possible."

The Pentagon commission recommended that future 20-year veterans receive a pension of about 35 percent of their basic pay, instead of the current 50 percent. They also would receive annual increases of only three-quarters the inflation rate, but would be compensated by a lump-sum retirement bonus.

The Grace commission, a presidential panel, said a military pension "costs more than six times as much as the better private-sector plans" and recommended that no benefits be paid until age 62.

The Congressional Budget Office found that the Pentagon panel's proposal would save $1.2 billion over five years and would increase retention of career military officers by 5 percent. The Grace plan would save $2.2 billion over five years, but would cause 11 percent of career officers to leave, the CBO said.