Age alone can hardly be Sen. Russell B. Long's reason for retiring. At the age of 66, he is far from elderly by senatorial standards. We in fact don't know why he decided to go. But we do know that life in the Senate must be less amusing for him than it used to be. Dwelling in the minority can only be dull after 16 years as chairman of the Finance Committee, but there's more to it. A rebellion has developed, in Congress and through the country, against the kind of tax legislation of which Sen. Long is a master. The movement for tax simplification is a counterattack against the federal tax code as it stands, and as Sen. Long richly contributed to it.

From 1965 to 1981, as the chairman of the committee that writes revenue legislation, he had an extraordinary influence on the development of American tax law. To that work he devoted great intelligence, deep experience and bottomless cynicism. It was a style of legislation in which the special benefits -- the point of the game -- were most likely to be tucked inconspicuously away as a special exception to innocuous rules for other purposes.

He has taken good care of the industries of his state of Louisiana -- oil, gas and shipping prominent among them -- and a good many other states as well. He occasionally responds to the populist tradition; he has a genuine interest, for example, in the federal health insurance programs. But the income tax has been his abiding preoccupation. He views politics as the process of managing the competition among people who want things from their government -- usually things of a monetary nature. The way to do it is by giving out a tax preference here to be counterbalanced by another preference or two there -- and the less visibly, the better for everyone. One day some years ago, when the committee was working on the tax bill of the moment, a member complained that he couldn't follow what was going on. Chairman Long briskly replied that if all the senators insisted on knowing what was in the bill, the committee could never get it through.

The national exasperation with the present labyrinthine and opaque tax law is generating momentum for broad structural reform. The Treasury Department's current tax proposal is an attempt to reverse the long trend toward greater complexity. So is the Bradley-Gephardt bill. So is the Kemp- Kasten bill. The administration says that it hopes to see action this year.

Sen. Long will probably smile and tell his friends that it can't be accomplished because, when it comes to actual choices, most people want to preserve their own cherished tax preferences regardless of the costs to simplicity and equity. It would be too bad if he turned out to be right.