Defense Secretary Harold Brown had some bad news for President-elect Ronald Reagan yesterday and some good for the incoming chief of the Pentagon, Caspar W. Weinberger.

The bad news, Brown said at a breakfast with reporters, is that "at some point in the mid-1980s conscription may become necessary, but I don't see that now." The reason, Brown said, is that the youth population is projected to decline to the point that the military services may not be able to recruit all the people they will need later in the decade.

The good news for Weinberger, who will come to the job of defense secretary with the reputation of being a budget-cutter, is that there is "lots of money" in defense programs that could be trimmed without impairing national defense. Brown warned, however, that stopping procurement of outmoded weapons is easier said than done because of the political pressures to keep them in production.

Although Brown in his four-year tenure has called the all-volunteer force a success and resisted efforts to return to conscription, he noted that this position will be harder to maintain as the population of young people declines.

The Census Bureau estimates there are 10,758,000 males aged 17 to 21 in the U.S. population today, the so-called "target group" for military recruiters. By 1984, the total will dip to 9.9 million and to 9 million by 1990.

For the moment, however, recruiters are finding plenty of qualified men and women to fill the ranks of the Army, Navy, Air Force and Marine Corps. The combination of few jobs in civilian life for many people in this age group and the extra pay and benefits recently approved by Congress for military people is credited with changing attitudes toward a military career.

Undersecretary of the Navy Robert J. Murray, for example, said in an interview yesterday that thousands of Navy people, including those with critically needed skills, perceive a favorable change in military life and are opting to stay in the service.

On top of the 11.7 percent pay hike recently enacted for service people, Congress this week voted extra pay for sailors on sea duty and officers running nuclear submarines. The Navy considers keeping trained people its number one personnel problem, not signing up enough volunteers.