This is the year America's baby boom generation begins to turn 40. Never has there been such a self-conscious and defined generation in our history as that born between 1946 and 1956.

According to Jeff Greenfield, a television commentator who worked in Robert F. Kennedy's 1968 presidential campaign, this generation had a sense of being different -- of being subversives in an adult society -- as early as the 1950s, even before its members reached puberty. Unlike a generation that had enjoyed Benny Goodman and Glenn Miller with its elders, this one had its own music in the nascent rock and roll, which it treasured from the start because the baby boomers knew it was going to drive the grown folks bonkers.

We weren't very far into the 1960s before it became apparent that this group was different and that it considered itself special. I first encountered it face-to-face in the 1968 McCarthy presidential campaign. Some were veterans of the civil rights movement and brought the moral fervor of that crusade to the anti-Vietnam war movement. In the course of a conversation, some of them asked if I thought they were different from other young people I had known previously, if they weren't, well, more idealistic and moral.

I recall noting how many of them there were, marveling at how naturally they assumed that they could change the world and admitting that, yes, they were different.

There are a number of ironies about the Boomers as they settle into middle age, however. Nobody noticed when the Depression Generation or the World War II Generation, itself a postwar prosperity baby-boom group, turned 40, or 50, or 60; unlike the Boomers these generations are identified by the major historical event of the time they came of age. No one snapped to and came to attention, God knows, when the Silent Generation, which immediately preceded the Boomers, reached an age milestone.

Which was just fine, because the only time anyone took note of us was to sneer at how conformist we were, how passive, how timid, how . . . silent we were. No two generations offer a starker contrast than the Silents and the Boomers, as even the names suggest.

The Boomers are the largest population bulge in the nation's history -- the "pig in the python" as demographers indelicately refer to them. They were born during the longest period of economic growth and affluence and greatest national confidence in the country's history.

As a result, to them, everything was possible -- instantaneously. No other group had such high expectations. Many of them also had the idea that their happy adolescence could be prolonged indefinitely.

Not so the Silent Generation. We couldn't wait to become adults, and had modest expectations, for excellent reason.

We have the distinction of being the product of the nation's lowest birthrate in history. We were born during the depth of the Great Depression, and for all we heard during our formative years it sounded as though it might well be downhill from there.

In college, I had a political science professor who had been a politically active student in the '30s -- it's remarkable how similar their rhetoric was to that of the '60s -- who used to harangue us about our lack of commitment.

I remember staring at him in disbelief. I was then eyeball-to-eyeball with Selective Service Board No. 140, with both of us having the Korean War very much on our minds. Here I am, along with my contemporaries, I thought, concerned with survival and this guy is in a snit because we're not all cranked up to help with the perpetuation of the New Deal.

But while the Boomers had higher expectations and assumptions, life hasn't been any easier for them.

First of all, the Korean War was much shorter than Vietnam, and because the manpower pool was so small there was a rough equity in the draft. Almost all of us went eventually, and we weren't faced with the agonizing choice between going to an unpopular war and finding a way to beat the draft.

A great irony of the Boomer generation, with its sense of identity, is that it was one of the most openly divided on class lines of any in our history, because of the draft. The size of the generation meant that not everyone had to serve, and we know generally who went to Vietnam and who didn't.

Another irony is that the Boomer generation, which is remembered for its political activism, now is indistinguishable from its predecessors in its political behavior. As with young people always, its turnout is the lowest of any age group -- a study of the 1984 elections shows that 82 percent of those who rarely vote are 40 or younger and that they voted Republican and Democratic, Reagan and Mondale, in about the same proportions as everyone else. Those under 30, in fact, most heavily supported Reagan.

The leaders of both parties assume that whichever captures the allegiance of a majority of the Boomer generation will be the majority party into the 21st century. There are indications, however, that the Boomers are wary about both parties, and many are likely to remain independent.

Despite this, they have made their mark socially, culturally and politically, but the passage of time is a great equalizer.

Welcome to middle age, Boomers. It's not all that bad, given the alternative.