Selective Service officials said yesterday that the Army will be drafting 21-year-olds by the end of this year--which means that undergraduates, especially seniors, may have to re-examine their plans for the future.

The College's Office of Graduate and career Plans in the past has recommended to many students that they wait a year before deciding whether to enter a graduate school. The drop in the average are of draftees--a result of President Kennedy's order exempting married men from the draft--means that graduating seniors will have to chose immediately between grad school and the Army.

A spokesman for national Selective Service headquarters in Washington said yesterday that the Army would not defer a student who takes a year off between college and graduate school, even if he has been accepted by a grad school. "They're on their own after they leave college, and they run a good chance of being drafted," he said.

Students who take a year's leave of absence from the College may also be affected by the drop in the age of draftees. Although the College ordinarily gives such students a letter saying that they will be readmitted after a year, draft officials say they will not grant deferments to students who leave the College.

Draft officials emphasized that no students were being drafted out of college, except for "a few people who don't keep in touch with their draft boards." They recommended strongly that students apply for 2-S deferments.

Before President Kennedy issued the executive order exempting married men, the Army drafted few men under 23. The decrease in the manpower pool and a drop in the rate of re-enlistment have forced draft quotas up from 9000 a month before the executive order to a peak of 17,000 this November.

The quota is expected to level off at 12,000 to 14,000 this spring, but draft officials said they saw no prospect of an increase in the age of draftees. "We expect we'll be drafting 21-year-olds un- til there's another change in the law," the Washington spokesman, said.

Not only are men being called up for pre-induction physicals sooner, but they are also entering the Army more quickly than before. "Last year when we'd call a boy for his physical, he might have a five or six months wait before being drafted. Now if a boy takes his physical in January he'll be in the Army by February, or March at the latest," a Boston draft board official said.

This means that a man, who is called for a pre-induction physical in the near future probably will not have the option of entering National Guard or Reserve six-month programs, which ordinarily require a wait of several months after enlisting.

The Boston official added that his own office was calling up a great number of men in an attempt to build up a larger pool. He said the five-month waiting period might be reinstituted in the near future