His mother said, "Oh, my God!" when she heard he intended to run for Congress, but now Mrs. Shannon's little boy James has his own office on Capitol Hill. And as the youngest federal legislator, Shannon doesn't mind the title "Kid Congressman."

"You stand out; people know who you are," says the Andover and Johns Hopkinseducated congressman who turned 27 earlier this month. Shannon -- a Democrat from a district near Boston -- is the only congressman born since 1950. When elected last year at age 26, he was a mere year over the minimum constitutional age for his office.

Since he was 8 years old, when his father, a physician, took him to rallies for John Kennedy, Shannon has been drawn toward politics.The magnet was Washington, which i, why he attended college in Baltimore. Impatient to get his name on a ballot somewhere, he earned his degree in political science in three years and immediately pursued a law degree at George Washington University.

Shannon was a political junkie on Capitol Hill who spent his free hours lobbying against America's involvement in Vietnam, interviewing members of the Massachusetts congressional delegation, and writing a thesis on Tip O'Neill. In that last project Shannon predicted O'Neill would never become House speaker; it was, of course, Speaker O'Neill who this year made Shannon the youngest member of the House Ways and Means Committee since 1804.

"I had kind of been thinking of how to get elected to the House," says Shannon of his student days in the Washington-Baltimore area. He returned to his hometown of Lawrence, set up a law practice that never did very well, and began thinking politics. "I keyed on a state senate seat and in 1976 lost by nine votes to the incumbent [who was 50 years older than Shannon]. I thought I was the unluckiest guy in the world."

His luck changed. A House seat opened unexpectedly when Paul Tsongas decided to run for Edward Brooke's Senate spot. Shannon won in a crowded primary and then handily defeated the mayor of Lowell in the general election. While his age was not a major issue, his stand against Proposition 13-like proposals, his belief that the government ought not meddle in the abortion issue, and his advocacy of mandatory wage-price controls did set him apart.

Shannon does not have a lean and hungry look despite his lifelong lust for office. He is serious and amiable and his early success surprises him.

"I arrived here never having done anything longer than a stretch of four years," he says. "I can't envision doing the same thing when I'm 40 years old. The worst that can happen is that I'll have done what I wanted to do for at least one term. Now that I've done it, I can think about other things... I can think about lying on a beach for six months someday. That never was a possibility before."

Shannon's wife of six years, who's completing her Ph.D. in French history at Boston University, just joined him here.

One other congressman is delighted with Shannon's youth. He's Mike Synar, a 28-year-old Oklahoma Democrat, and the second youngest House member. (The dean of his state's delegation was a congressman two years before Synar was born.)

"I told Jim I was awfully glad he was here because he's the youngest," Synar says. "And even though it's wonderful title to have, I'm glad someone is younger than I am. I told him at a White House breakfast that he should just keep trying and that someday he might be my age." CAPTION: Picture, no caption, By Bill Snead