If you buy Sen. Orrin Hatch's assumption that the rising minimum wage has priced teen-agers out of the labor market, then it's hard to resist his solution.

The Utah Republican says he will shortly introduce legislation to permit employers covered by the minimum-wage law to hire teen-agers at 75 percent of the adult minimum.

The result, he predicts, will be to render teen-agers more employable, thereby reducing the disastrous levels of youghful joblessness.

The proposal is sure to be bitterly opposed by labor unions, liberals and blacks. But why?

Are these liberals opposed to doing something concrete about the youthful unemployment that is threatening the tranquillity of the cities? Can blacks argue that it is better for a youngster to be unemployed at $3.35 (the minimum wage beginning next January) than to be employed at $2.50? Is Labor's resistance nothing more than an attempt to save jobs for union members, at the expense of teen-agers? What are the arguments against the so-called youth differential?

Some of the arguments are fairly obvious, and Hatch proposes to meet some of them. One objection, for instance, is that employers will hire teen-agers at the sub-minimum (75 percent of the adult minimum, in Hatch's proposal), then replace them with a fresh crop of teen-agers as the original cohort reaches age 20. Another is the fear that fathers will lost their jobs to their own sons.

Hatch intends to meet these objections by requiring payment of the full minimum wage after a six-month training period, or when the worker reaches age 20. Meanwhile, the youngsters will have had the chance to prove themselves.

Presumably, employers would find it more economical to keep experienced workers, even at the full minimum, than to keep training new ones at the sub-minimum.

Meanwhile, there are all these formerly jobless youngsters now going to work every day.

That, at least, is what might happen if Hatch's basic assumption is correct. The problem is, nobody can demonstrate that it is correct, and there are a good many economists and labor experts who would argue that it isn't.

Even Hatch himself seems to have his doubts.

"Everyone knows," he said in a recent interview, "that when the minimum wage goes up to $3.35 an hour, thousands -- no hundreds of thousands -- of kids will lose their jobs because businesses just aren't going to pay that much for young people who are only worth $2.50 an hour."

How's that again? There are these "hundreds of thousands" of youngsters already employed at the current minimum of $3.10 -- surely a number of them with at least six months on the job -- and they are going to be laid off when the minimum wage goes up by a quarter an hour? Even when their replacements would have to be paid $3.35 by the time they became fully trained?

Sorry, senator, it doesn't compute. The sub-minimum would do nothing for the youngsters already employed. Nor would it be likely to do much for their jobless counterparts if employers know that, after six months, they'll have to get the full minimum.

The only way it could work, it seems to me, is for the youth differential to be permanent -- at least until the worker is no longer a youth. And that would produce either of two results, neither of them terribly reassuring. Either the 20-year-olds would be laid off as no longer worth their pay, and their jobs given to 17-year-olds, or there would be two groups of employees, equally experienced, doing the same exact job, but a differential pay, based solely on their age.

Try to sell you 17-year-old son on taking a counter job at McDonald's for $34 a week less than his 20-year-old co-worker, with every prospect that he will lose his job altogether when he reaches that magic age.

Even with these objections, the Hatch proposal might be worth patching up -- if it were true that the minimum wage is the major cause of youthful joblessness.

A number of people who have examined the problem think the causes lie elsewhere: in the fact that a growing number of the jobs available to teen-agers are in the suburbs, while the bulk of the jobless teen-agers are in the inner cities, and in the fact that a number of employers simply don't want black, inner-city youngsters around.

I suppose you could fix the second problem by setting pay scales low enough. But then the kid becomes a more attractive employee than his own father, with all the problems that entails.

I don't doubt the senator's good intentions, but maybe he'd better hatch up another scheme.