When he died last month at 92, Democrat Robert C. Byrd was the last serving member of the Senate born before 1920.
The man named Friday to replace him will become the first U.S. senator born in the 1970s.
Carte Goodwin, 36, was appointed by West Virginia Gov. Joe Manchin III to serve until a special election this November decides who will hold the seat.
Goodwin, the former chief legal counsel to the Democratic governor, gives Democrats in the Senate a crucial 59th vote just as they prepare to push for passage of a key priority, an extension of unemployment benefits.
Senate leaders will move quickly to make use of Goodwin's vote. They are planning to have a brief ceremony Tuesday afternoon to swear in their newest and youngest member. Moments later, the Senate will hold a key test vote on a Democratic-backed proposal to extend jobless benefits.
Goodwin's time at the Capitol is expected to be brief. Manchin soon may sign pending legislation authorizing a special election to allow voters to choose a new senator to complete Byrd's unexpired term and serve until 2013. Manchin, 62, has said he plans to run in that election.
Byrd was 57 and in his 21st year in Congress when Goodwin was born in 1974 — a generational chasm that highlights the Senate's longstanding reputation as a chamber of elders.
Before Byrd's death, the average age of senators was more than 63, the highest in history, according to the Congressional Research Service.
House members have an average age of 57. That chamber has several members born in the 1970s and at least one — Rep. Aaron Schock (R-Ill.) — who was born in the 1980s.
Goodwin said he was humbled by his selection. He said that while he could not replace Byrd, he would try "to emulate his work ethic and his commitment to the law, the Constitution and this great state."
Goodwin supplants Sen. George LeMieux (R-Fla.) as the youngest Senate member. LeMieux, 41, was born in 1969.
However, Goodwin is not the youngest member ever to serve. Among current senators, Patrick Leahy (D-Vt.), now 70, was youngest when he joined the Senate at age 34 in 1974, the year Goodwin was born.
West Virginia Republicans criticized Manchin's choice of Goodwin as the appointment of a "longtime loyalist."
"This appointment demonstrates the need to have voters have a say in who represents them in the U.S. Senate," said Troy Berman, executive director of the state GOP.
Both parties are gearing up for the coming special election. Those familiar with pending legislation said that the filing period for an earlier special primary election could begin next week. Republicans hope their candidate will be U.S. Rep. Shelley Moore Capito, who has not divulged her plans.