The late afternoon shadow of the fence falls across the boy's back. Sitting here by himself on the south levy of the Tijuana River -- the Mexican side of the river -- he is thinking about crossing. Just a step to swing around the post of the half-built barrier and he would be in U.S. territory.

But the Border Patrol agent is watching from the pale green Ramcharger a few yards away. It wouldn't be worth the trouble to move now. Maybe tonight.

There will be hundreds, possibly thousands, crossing this stretch of border then: young men and old women, little children, grandmothers with their hair in long gray braids and their cotton dresses pulled up to their knees.

They will stalk across the arid hillsides to the east, where the densely packed houses of Tijuana give way to the suburban openness of San Ysidro, Calif.Then they will slog through the mud here in the river, vaulting fences and crawling through drainage tunnels to hide out in trailer parks and in the prim alleys between development houses. t

Smugglers will prowl the U.S. streets waiting to pick up their human contraband, and from atop hillsides and overpasses the U.S. Border Patrol will be waiting and watching, with binoculars and infrared scopes, to call in men on horseback, in patrol cars or on foot -- whatever it takes and whatever they have to catch as many of these illegal aliens as possible.

This border is a rough part of the world, a high-pressure vent where an overflowing, largely impoverished society pushes up against the richest nation on earth.A lot of unpleasant things happen along the line. People are beaten and die, on both sides. There are bandits and drug smugglers, armed and often ready to kill.

But in a sense, it is the boy here sitting on the levy that scares the policymakers of the United States most.

Javier is 14 years old. According to official Mexican statistics, almost half of this country's population is his age or younger. The long-term future of their nation, with its copious quantities of oil, is bright. But for most of these 30 million children, the next few years look grim. Projects are under way to create more jobs for them, to use oil money to produce enough food for them, to build a society that no longer needs to be called developing where they can thrive as well as survive.

But Mexico's 92 million population is expected to double by the year 2000. Almost certainly many like Javier will be left out of the country's future and will have to look for their own somewhere else. Most likely they will look in the United States. As Americans suddenly become aware that they no longer have even enough resources to satisfy their current demands, the prospect of thousands, perhaps millions of Javiers crossing into the United States terrifies them.

There are already plenty of laws on the books to keep Javier from entering, but with 2,000 miles of border, the gaps in the fence sought out by the poor are even vaster than the loopholes in the law that can be used by the rich.

Javier's father died last month in Guadelajara, and two weeks ago, making his way alone, Javier came here to Tijuana. He found odd jobs and lives in a dilapidated boarding house. His uncle, a smuggler, helps him out a little bit.

Javier is not sure where he will live, or how he will get a job if he makes it across the line. He has one sister in Long Beach, Calif., and another in the state of Washington, but he is uncertain how to find them. His sisters, as far as he knows, are not even aware that their father has died.

Because Javier wants to get a job in the United States, and perhaps just because he is 14, he is trying hard to look like a man. He and his friends have etched crude tattoos onto both his arms. Pulling up his shirt, he proudly shows a coiled rattlesnake carved onto his back. The scratches of the pocket knife used to imbed the ink are still visible.

If he had started school this month, Javier would be in sixth grade. Education is a good thing, he told a visitor who sat down beside him on the levy. It would be good, maybe, to go to school in the United States. But right now, he said, he has to work.

It is so hard to find work that almost everyone crosses this border. Not only the young have difficulty finding jobs in Mexico. By some estimates more than 40 percent of the country's adult work force is underemployed and without job security.

Jorge and Maria, sitting on another stretch of the levy, are in their mid-40s. They came to Tijuana from the state of Michoacan a month ago, leaving their children behind. Jorge has found no work here and, he too, is thinking about crossing to the United States.

There's not much work where there's no water, said Jorge, and where he lived there was almost none this year.

Maria is sewing quietly in the cool breeze along the levy, and Jorge has been literally testing the waters of the shallow stream beneath them. His pants legs are rolled up, and there is mud on his bare feet. On the north levy directly across from him a Border Patrol agent is sitting on the hood of his car.

"I walked down there," Jorge said, pointing, "and the man asked me where I was going. 'Going back,' I told him." Maria Laughed.

There are some studies that indicate the Mexicans who actually cross the border will come back to their homeland. They are less immigrants than migrants, crossing to find short-term jobs, then coming home again, according to these findings.

Jorge Bustamante, director of a vast research program sponsored by the Mexican government, estimates that the number of Mexican illegal aliens who actually stay permanently in the United States may be as small as 5 percent. Although his study has not been released yet, some sources familiar with it expect it will show something less than 1 million illegal Mexican immigrants living in the United States at any one time. Census bureau estimates in the United States, however, place the total illegal immigrant population closer to 3.5 million.

Already Bustamante's figures, or at least reports of them, are under attack. Even if they prove true for a given period in time, U.S. officials concerned with immigration doubt that they will remain true for long.

Mexico is not the only source of immigrants in the hemisphere. Other pressures are building as well among the tiny Central American countries to the south. While over the long run the process of radical social change may lead to less cause for Central Americans to come to the United States, for the moment it has apparently more than doubled the number of Central Americans crossing the border here at Tijuana.