NEW YORK (AP) _ Freshman applications are down at top colleges around the country for the first time in years, the likely outcome of a dwindling number of high school graduates and ever-higher tuitions, admissions officers say.

The decline in graduating high school students is considered the prime culprit, according to more than a dozen admissions officers interviewed since Feb. 1, the deadline for fall freshman applications at many of the nation's most competitive schools.

''Since this is happening to everyone, the best guess is that it's demographics,'' said Michael C. Behnke, admissions director of the Massachusetts Institute of Technology, where applications are down 10 percent from a year ago.

But some officials suspect that students and their families may finally be reacting to tuition increases that have consistently exceeded the nation's overall inflation rate throughout the 1980s.

''I think there's a great possibility that tuitions are a part of it,'' said Linda Davis Taylor, admissions director of Amherst College, where applications are off by 4 percent from last year. ''I don't know of any topic that's being discussed more by boards of trustees.''

The number of high school seniors has dropped steadily from 3 million in 1980 to 2.76 million in 1988, and is expected to bottom out at 2.44 million by 1992.

An unexpected increase in the ranks of older students and higher percentages of high school students attending college helped offset that decline in the number of 18-year-olds. Thus, to the surprise of many, college enrollments grew from 12.1 million to 12.5 million since 1980, and were up at 54 percent of the nation's colleges and universities, according to an annual survey by the American Council on Education.

But college officialsbelieve that the drop in applications for next fall's freshman class is the first sign that the long-expected decline in enrollment is about to arrive.

Harvard University spokesman Peter Costa estimates a 5-10 percent drop in applications, the first such decline in at least two decades. Brown University and most other Ivy League schools are reporting similar declines.

Stanford University's fall applications have dropped 6 percent, from 15,828 to 14,869, according to Lynne Madison, assistant dean of undergraduate admissions.

At University of California at Berkeley, applications are also down, from 21,944 to 20,835 for next fall's entering class.

''I consider this a yellow light. We all kind of anticipated this,'' said Bradley Quin, admissions director of Lafayette College in Easton, Pa., where applications are down about 5 percent. ''I don't mean to imply that we're not concerned. This is a competitive business, and it's going to make it that much harder to increase the quality of our classes.''

Apart from the population changes, school officials say students may also be cutting down on the number of schools they apply to following a decade in which multiple applications increased sharply.

An annual survey of entering freshmen by the American Council on Education and the University of California at Los Angeles found that a record 37 percent applied to at least three colleges in fall 1988, compared with 26 percent in 1980.

That increase has generally been attributed to students wanting to improve their odds of getting into a top school, and to shopping around for the best financial aid deals as tuitions have soared.

But students are apparently starting to think twice about submitting a dozen or more applications now that application fees have hit $40, $50 and more.

''When you start talking about $50 fees, you get more selective,'' said Sanford Rivers, associate director of admissions at Carnegie Mellon University in Pittsburgh.