In theory, Howard Fast has hold of a solid commercial idea in "The Immigrant's Daughter": Ride the demographic wave and target a book for the fastest growing segment of the population -- the over-sixties.

Give them a new kind of character to identify with. Enough of this front-porch rocking into the sunset years. Enough of those once-vital characters who are just excuses for novel-length flashbacks about the empires they built and the wastrels they begat. Since women are in the majority in this age group, make this new character a woman who has got a lot of living to do. Better yet, make her a woman your readers are familiar with.

Reenter headstrong Barbara Lavette for the fifth book in the successful series that began with her father, Dan Lavette, the life force of "The Immigrants."

Now 60-plus, Barbara Lavette is still courted by younger admirers; still firm of breast and buttocks (or so we're told more than once); still eager for men and aching for them (or so we're told more than twice) after two husbands and two lovers.

She retains her credentials as a Renaissance woman: rich (heiress to several fortunes), independent (World War II correspondent), rebellious (jailed for defying a congressional committee in the McCarthy era, organizer against the Vietnam war) and famous (successful author).

And now she's about to add to them by running for Congress as a feminist candidate and by taking on a dangerous reporting assignment in revolution-torn El Salvador -- because "there are better ways to go than to dry into nothingness and become what they so euphemistically call a senior citizen. To hell with that! I need work, because if I don't work, I'm going to die -- the wrong way."

It's more than a little discomforting. Our heroes and heroines are supposed to know when to leave the stage: Just how old can we let Travis McGee get? But now's the time to break the mold. So set it all against the rich backdrop of a family saga, put it in the hands of a veteran like Fast -- he's written more than 40 books, including "Spartacus" and "Freedom Road" -- and you should have the ingredients for a sprawling winner.

That's the theory.

In practice, however, Fast is so busy delivering his lessons that he squanders his opportunities to entertain. From Michener's "Hawaii" to McCullough's "The Thorn Birds" to Clavell's "Noble House," few themes have been so satisfying as the life cycle of a family -- the reminder that we are part of someone other, that we reach beyond death, theirs and ours. But here the other family members seem almost nuisances, characters to be dealt with or disposed of dutifully so the author can get on with the lectures.

On being an older woman:

" 'Sure I'm angry. I've been a widow for almost seven years. Try being a widow some time. Try looking at a telephone that stops ringing, that doesn't ring for a week at a time. Try being invited to a dinner party where you know damn well that it's poor Barbara, and it's just too long since we invited her, and try shopping around in a world that doesn't want you or need you! But I don't whine and whimper about it.' "

" 'Here I am, sound of limb and reasonably clear mentally, and most of the time I feel that I'm looking at a world that has ushered me out. Damn it, I'm not ready to be ushered out.' "

On U.S. involvement in El Salvador:

" 'Without your help, the death squads would not last a month. If your government would only leave us alone, we could finally build something good here; but as long as you arm these crazy monsters who kill us as if we were animals, as long you support them, we must fight you. We can't win, but at least we live. If we lay down our arms, there will be such a slaughter as the continent has never seen . . ."

Which brings us to the dialogue -- it's as wooden as the characters. And that's disastrous in a book where people hardly seem to do anything but talk. "The Immigrant's Daughter" is supposed to be an affirmation of life; instead it seems like an eternity until it's over.

Even for the many fans who can't get enough of Howard Fast, this may be too much -- of too little.