MR. REAGAN'S advisers are right to give immediate attention to Social Security -- and they are right to be apprehensive. Cuts in future Social Security benefits are no longer altogether unthinkable, but very few people have actually thought about them. Congress, until now, has behaved as though the only question were to find more money for more benefits. The rise in the payroll tax on Jan. 1 will be followed by well-founded hints and warnings that it isn't enough. There's more going on here than another routine campaign for another routine tax increase.

The Social Security system has now reached maturity, in the sense that nearly all Americans reaching the age of 65 are entitled to benefits. In 1950, there were 14 people who worked and paid payroll taxes to support each person drawing a Social Security check. By 1970, the ratio was four taxpayers to each beneficiary. Next year, with 36 million Americans getting Social Security benefits, the ratio will be less than three taxpayers to each beneficiary. But that's only part of the change overtaking the system.

In 1977, Congress sharply increased the maximum tax and the maximum benefit. As the insurance companies keep plaintively saying, Social Security was orginally supposed to provide only bare-bones benefits to protect the elderly from destitution. But middle-class Americans have perceived that Social Security is the only kind of pension that can reliably insure them against inflation. By law, Social Security benefits rise with prices. That's something that no annuity or private pension plan can promise. If the present inflation continues and if it goes higher, the private retirement plans will inevitably become less attractive. People will turn to Social Security not as a supplement, but as the mainstay of their retirement income.

It's an instructive example of the way in which inflation expands the reponsibilities of the federal government, as the only institution that can protect income against it. But this protection in turn has serious consequences. Insurance companies and pension funds are major sources of investment capital. As you pay insurance premiums toward a retirement plan, the insurance company holds that money and invests it. Social Security is different. The Social Security taxes that you pay this year are not held, but are immediately paid out to this year's beneficiaries. There's very little to invest. If Social Security continues to evolve toward a national pension plan, replacing the private plans, one of the economy's great conduits of savings and investment will disappear.

To stabilize the Social Security system, and to hold it to its present role, the Reagan administration will first have to slow down inflation. But that's one subject on which the still uninaugurated administration does not seem to have its strategy yet.