The Metro transit authority is weighing new measures to improve bus service, including a plan for getting rid of old, trouble-prone buses and a proposal for a "model" garage to serve Southeast Washington.

In an initial move, the Metro board adopted a policy yesterday aimed at reducing the average of age of its buses to six years. Metro's bus fleet now averages more than 10 years old, with nearly 700 buses from 11 to 22 years old.

Older buses are far more likely to incur breakdowns requiring costly repairs than newer ones, according to a Metro study.

Under the board's policy, one-twelfth of the authority's buses would be replaced every year. If the transit agency maintains a fleet of 1,800 buses, as some officials proposed, a total of 150 buses would have to be purchased annually.

Officials cautioned, however, that it is uncertain whether the transit system can meet this goal because it would require millions of dollars in federal and local government funds every year. A new bus currently costs about $175,000, officials said.

Under the proposal for improving Metro's Southeast garage, buses would undergo special maintenance inspections, including oil analyses designed to detect engine defects before breakdowns occur.

More than 100 new or freshly rehabilitated buses would be based at the garage in the next few months.

Buses would also be required to be pulled out of service for regular maintenance after every 10 hours of operation. Buses dispatched from the Southeast garage now average nearly 12 hours of service a day, the highest rate in the system.

The proposal, which has not yet been considered by Metro's board, is aimed at using the Southeast garage as a model for testing whether more intensive maintenance leads to fewer breakdowns. At a committee meeting, some board members expressed concern about the plan, partly because it might reduce the number of new buses available for routes in Maryland and Virginia.

"If you're going to give us the old buses and take our mechanics, I don't know what the effect of that is going to be," said Cleatus E. Barnett, a Montgomery County board member.

Buses from the Southeast garage operate mainly on routes serving Anacostia and other low-income areas of Southeast. The garage has long been regarded as one of Metro's most dilapidated facilities, and buses based there have one of the highest breakdown rates in the system. Officials attribute the breakdowns partly to the buses' heavy use on congested roads.

For several years, the transit agency has sought to find a site where it could build a new garage to replace the outmoded structure on M Street SE. It is now studying a possible site at Buzzard Point in Southwest.

In another development, the board ratified plans for starting rail service on the Yellow Line to Alexandria and Fairfax County on Dec. 17, an extension that had been delayed by a rail car shortage.

Changes will also be put into effect on the Blue and Orange lines. Starting Dec. 17, the Blue Line will operate in both directions between National Airport and Addison Road, and the Orange Line will run in both directions between Ballston and New Carrollton.

In a separate move, the board adopted proposals for starting construction on several portions of the planned 101-mile rail system in Maryland and Virginia that have not been endorsed by the Reagan administration. It is unclear how federal officials will respond to the plan.