A Brookings Institution study says that the U.S. military could be made more productive - at a $300 million saving - by retaining more older military personnel and enlisting fewer recruits.

The report released yesterday contradicts a tradition of the military - the recruiting of young men and women while using hefty pensions and bonuses to entice older personnel into early retirement.

Most military personnel now retire after 20 years of service, and most go into the private sector to collect salaries on top of their pensions. The retirement system was designed to create high turnover.

But, the study says, in light of today's highly technical military, where intercontinental missiles have replaced trench soldiers and where wars can be conducted by pushing buttons on consoles, there is no longer such a need for "youth and vigor."

The study concludes that retaining older, more experienced personnel is important for the modern military. It recommends increasing the minimum length of service of military personnel and eliminating the pension and fringe benefit package that has been used to make retirement after 20 years attractive.

Instead, the study recommends the military use increased pay "to promote retention and enrich the experience mix of the military labor force.

"Since military pay is now in line with federal civilian pay, it would be reasonable to allow military personnel to retire in much the same way their civiliam counterparts do . . .," the study says.

The study says that retaining experienced personnel also will mean a financial savings, because "a young force is an inexperience force. . . . The appropriateness of a military work force in which close to 40 percent of the employes are apprentices, helpers or trainees is called into question."

Study found the major burdens of maintaining a young military to be the increased recruitment costs since the institution of the volunteer Army, the costs of training programs and the costs of supporting new recruits with families and dependents.

"Unlike the armed forces of earlier eras that were dominated by combat operatives-infantrymen, tank crews, artillerymen, aircrewmen and fighting ships' companies-the vast majority of military personnel today are involved in supporting the combat mission," the study found.

A presidential commission is studying the military retirement system, an the Carter administration is expected to suggest revisions of the system based on the commission's findings.