One of TV's most loquacious analysts is happiest when teen-agers are doing most of the talking. The ubiquitous Jeff Greenfield has appeared on TV most recently as a media and political analyst for ''CBS Morning News'' and ''CBS News Sunday Morning.'' Before that, you may remember him as a regular on William F. Buckley's ''Firing Line'' and on PBS's ''We Interrupt This Week.''
He has recently become a regular on ABC's ''Nightline'' and ''Good Morning America.'' In future weeks he will be functioning as an ABC analyst of the forthcoming elections as well.
But unless you have been a viewer of ''Why in the World'' (PBS, in most areas Tuesdays and Thursdays during daytime to high schools; in the evenings or on weekends for general audiences; check local listings), you have not seen Jeff Greenfield glowing with pride in the new generation of youngsters.
''The thrill for me of acting as host for the show,'' he says earnestly, ''is the fact that at a time when teen-agers are supposed to be obsessed with video games and punk rock, twice a week we manage to get a bunch of smart high school kids for each show to talk intelligently about real issues in a nonpatronizing way.
''Recently we've tackled the role of the church in South and Central America, labor-management problems in the '80s, the art of political packaging, ethics in news, the College Board exams.
''Listen. There was a time when I was a teen-ager that there were lots of shows in which young people played a part - 'The New York Times Youth Forum,' 'College News Conference,' and 'Youth Wants to Know.' Now there is nothing around like that . . . except for 'Why in the World.' ''
There's a twinkle in his eye as he explains: ''I know we're not going to blow 'Thorn Birds' off the air. But it's fun for me . . . and for the kids. And if there are some youngsters out there who watch the show and realize that there is a forum for kids of their age to talk about public-policy issues intelligently, I believe that's a very useful thing and I am happy to have been involved.''
Jeff, a 1967 graduate of the Yale Law School, has authored or co-authored nine books. He has two youngsters of his own, not quite in their teens. And he is just a bit disturbed that his new job at ABC may not allow him to continue with ''Why in the World,'' which is more or less a philanthropic activity for him. After only a few more weeks of original programs, the show will continue through May on reruns.
Then comes the traditional wait for a funding OK for the future. According to executive producer Barbara Barnes, it is expected that General Motors will continue to fund the show for its third season. WNET/NY will continue to be associated with the show, with one show a week taped in Los Angeles, the other in New York City. This will allow students, teachers, and experts from both areas to take part.
Does Jeff Greenfield find that the bright teen-agers who are searched out in many local schools tend to be disenchanted with television?
''No,'' he says, ''but I find that on the whole TV is not the dominant factor in their lives. They use it, watch it for fun, but these are kids who have discovered that there is a world beyond the picture tube. Of course, they may very well have discovered that world while watching TV. . . .
''In a sense,'' he says just a bit nostalgically, ''they remind me of myself not too long ago. I was a public affairs junkie - I went to every mock convention, political debate, student UN council, etc. So I'm not surprised when I discover how bright and aware these kids are. It's just nice to see that, despite so much disparagement by some segments of our population, the fact is that bright kids are still as involved in the world around them as bright kids have always been.''
The idea for ''Why in the World'' originated with Walter Cronkite, who felt that our society was not taking proper advantage of the superb teachers available in various areas of the country. The original idea was to bring those teachers to the TV screen to talk to high school classes whenever current events focused on their specialties. The idea was soon expanded to include nonacademic experts as well as teachers, the general public as well as high school students.
How does the show function now? ''I set it up, introduce the participants, give a brief news package to illustrate what we are talking about, and then I get out of the way,'' says Mr. Greenfield. ''I'm a big basketball fan, and the part of the game I hate most is when the referees take over and keep blowing the whistle - so I try not to act like a referee.''
According to Mr. Greenfield, some of his favorite recent shows have been the one in which Mike Wallace answered questions about TV newsmen, David Leiderman of David's Cookies talked about starting a new business, followed by a professor of business administration who gave tips on going into business in the present economy. In the past few weeks there has been a discussion on the stock market, and programs about black teen-age unemployment and our prison system.
''The level of discussion is grown-up,'' Mr. Greenfield points out. ''And what I hope most is that there will be a few kids out there who will see the show and say to themselves: 'Look at that - those kids are my age and are asking intelligent questions, taking part in intelligent discussions. Why shouldn't I do the same?' ''