The new House of Representatives in the 96th Congress will be younger, slightly more Republican and, if campaign rhetoric holds up, more conservative than its predecessor.

"Everyone campaigned as a conservative," said Becky Norton Dunlop, who watched House races for the American Conservative Union. "Only time will tell whether they will vote the way they talked," she added.

Republicans picked up 12 seats in last Tuesday's elections, giving them 159 members to the Democrats' 276. The average age of the 41 new Democrats and 36 new Republicans is 41, and the turnover will lower the average age of all 435 House members to 48.5, down 9.5 months from 1977.

Richard Conlon, staff director of the liberal House Democratic Study Group, said the effect of a Republican pickup might make the pro-conservative swing stronger than numbers suggest.

"The defeat of a dozen liberals could cause others to hunker down and vote more conservatively next year," he said. It happened after the Republican comeback in 1966, he recalled.

On the other hand, Conlon noted, some of the Democrats' most unyielding elder conservatives have departed: Joe D. Waggonner of Louisiana, Robert L. F. Sikes of Florida, Omar Burleson, Olin E. Teague and W. R. Poage of Texas. Their replacements, while conservative, may be less so and in any case will have a better "peer relationship" with their younger colleagues who now make up most of the House, he said.

No common theme runs through the class of '78, such as many saw in the big group of 75 Democrats elected after Watergate in 1974. They banded together and shared in uncommon degree a desire to make government better, a distrust of big government and its grant-in-aid programs, a desire to open up the system to public view.

For one thing there are fewer Democrats (41) this time and more Republicans (36).

A Democratic campaign official called the new Democrats philsophically "a very mixed bag" and cautioned against trying to label them. Some classic conservatives in the South are being replaced with younger people not so conservative, he noted, while some nonsouthern liberals such as Michael Harrington (Mass.) and John E. Moss (Calif.) will be replaced by Democrats who, while regular party men, appear to be not as intensively committed to issues.

After a quick look at the 41 new Democrats, Conlon said they seem to break down this way: 10 conservatives, six from Texas; five southern moderates; 11 northern party regulars and about 15 "DSG-type reformers." He doubted they would make much change in the tendencies of the House. He considered the departure of the oldtime conservatives as being more important in that regard.

The 41 new Democrats come from 21 states. The largest number, seven, are Texans.

They average 41 years old, ranging from James Shannon, 26, a Massachusetts attorney, to Edward Stack, 68, a Florida county sheriff, who defeated a 65-year old Republican incumbent.

As usual a majority of the new Democrats worked their way up through lesser political offices. Seventeen served in state legislatures and include former house speakers from Connecticut and Minnesota.

Five held other elective office such as mayor or city councilman. Five had served on the staffs of House members. One is a former member - Peter Peyser of New York, who served six years as a Republican, lost a Senate bid and returned a Democrat.

Neither of the two new Democratic women members - Beverly Byron (Md.) nor Geraldine Ferraro (N.Y.) - had held elective office. Two of the three new black members - Mickey Leland (Tex.) and Julian Dixon (Calif.) - are members of state legislatures. The other new black - William Gray (Pa.) - is a Baptist minister. All 15 black members of the House next year, as now, are Democrats. Eleven of the 16 women in the new House are Democrats.

A contender for the title of most conservative new Democratic member would be Marvin Leath of Texas, 47-year-old banker and former aide to Poage, whom he will succeed. Congressional Quarterly reported that in 1964 Leath voted for conservative Republican Barry Goldwater over Texas' Lyndon B. Johnson for president.

The 36 new House Republicans have two general characteristics: conservative, particularly on spending issues, and experienced. Twenty of the 36 have been in government, usually a state legislature. Two worked for President Ford, one, Richard Cheney (Wyo.), as White House chief of staff, one, Tom Loeffler (Tex.), as a legislative assistant.

The average age of the group is 40.8, with the youngest 28, the oldest 55. The class has one woman, Olympia J. Snowe (Maine). It also has three doctors, 11 lawyers, a college professor, three investment couselors, two insurance executives, two university program managers, a dentist, an urban planner, a cable television executive and a mortician.

Fourteen of the new Republicans defeated incumbent Democrats.

The state with the largest number of the new Republicans in California (7), followed by Pennsylvania (4).

Dunlop of the ACU, who looked at the group in light of the issues they champion, said, "I think across the board 80 percent of them are solid. Not only that, we're excited about them being activists," members who are willing to fight for positions, not just passively casting a vote.

Typical of the new Republicans is James Sensenbrenner, 35, who won the Wisconsin district formerly held by Republican Rep. Robert Kasten. Kasten resigned to unsuccessfully run for governor.

Sensenbrenner, a conservative assistant minority leader in the state Senate, had a tough primary fight, narrowly beating a more moderate Republican, Susan Engeleiter, to win the nomination in the district that is made up of upper-class suburbs along Milwaukee's Lake Michigan shore.

In the general election campaign, his opponent, Matthew Flynn, a Yale-educated attorney, also took a conservative tone, arguing for heavy federal budget cuts.

Sensenbrenner endorsed the GOP Kemp-Roth bill, which would cut federal taxes a third over three years as a means of stimulating capital investment.

Sensenbrenner, who came up through traditional party politics, Republican clubs and then election to both state legislative bodies, is expected to fit easily into the legislative process and the Republican Party in the House when he gets here.

Californian Dan Lungren, 31, defeated one of the post-Watergate Democrats of the so-far largely reelected class of '74, Rep. Mark Hannaford.

Lungren, who former worked for Republican Sens. George Murphy (Calif.) and Bill Brock (Tenn.) and the Republican National Committee, narrowly lost to Hannaford in 1976.

This time, he took better advantage of the money he got from the national party, which had targeted Hannaford, and from corporate political action committees, and carefully studied the issues.

Though Hannaford had changed his voting pattern to the more conservative tone of his district and held numerous town meetings, Lungren successfully charged that Hannaford presented a conservative image in the district but voted liberal in Washington.

Snowe, 31, of Maine becomes the fifth Republican woman in the House. A former district office manager for Sen-elect William Cohen (R-Maine), Snowe was elected to the State Senate upon the death of her husband in 1973.

Unlike many of her new Republican colleagues, Snowe is expected to be moderate to liberal. In the state Senate, she concentrated on health care issues.