Alexi Giannoulias, the Democratic state treasurer of Illinois, was 4 years old when GOP Rep. Michael N. Castle was elected lieutenant governor of Delaware. But come 2011 the two men could serve together in the Senate.

Giannoulias, 33, is one of a handful of young candidates running to become a member of the what is now the oldest — in terms of average age — Senate in history. Kentucky’s Republican Secretary of State Trey Grayson, 37, and GOP former Florida state Speaker Marco Rubio, 38, are among the other under-40 Senate candidates whose 2010 bids have gained some national attention.

But while next year’s midterm electorate is likely to be older than in a presidential year, there’s no indication that being a younger candidate will be an obstacle for any of the 30-something crowd running.

“No one has said, ‘You’re too young to be a senator,’” Grayson said in an interview.

Indeed, it will be difficult to take issue with Grayson’s youth and inexperience because he’s in the middle of his second term in statewide office — and the other three main candidates in the race aren’t that much older than he is.

Giannoulias and Rubio aren’t political newcomers either. The Illinois Democrat was elected in 2006 at age 30 while Rubio was first elected to the state House when he was 29 and was Speaker by age 35.

With their political backgrounds, Giannoulias, Rubio and Grayson are in good company. Nine current senators were elected before the age of 40 and all of them had previous experience in elected office.

Among the Democrats, Blanche Lincoln of Arkansas, Christopher J. Dodd of Connecticut, Daniel K. Inouye of Hawaii and Max Baucus of Montana served in the House. Jeff Bingaman of New Mexico, Kent Conrad of North Dakota and Mark Pryor of Arkansas held statewide office, and Russ Feingold of Wisconsin served in the state legislature.

Patrick J. Leahy , D-Vt., who was state’s attorney for Chittenden County, was elected to the Senate in 1974 at age 34 — before Giannoulias was born — the youngest current senator at swearing-in.

Until this year, that distinction was held by Vice President Joseph R. Biden Jr. , who was elected to the Senate from Delaware at age 29 and turned 30 before being sworn in in January 1973.

The late Sen. Edward M. Kennedy , D-Mass., was elected at age 30. In fact, the governor appointed a placeholder, Benjamin Smith, after Kennedy’s brother, Sen. John F. Kennedy, was elected president in 1960 — because Ted Kennedy was then too young to serve.

According to the Senate Historical Office, the average age of senators at the beginning of the 111th Congress was 62.7 years. The average age in the 1st Congress was 47 and stayed in the 40s for all but three of the first 30 Congresses. More recently, the average age of senators has increased steadily every Congress over the past decade, and the last three Congresses have been the chamber’s oldest.

This cycle’s crop of young Senate candidates is balanced out by some political veterans such as 65-year-old Democrat Roxanne Conlin, a former president of the Association of Trial Lawyers of America, who is running in Iowa, and Castle, 70, who is running in Delaware. He’ll likely face state Attorney General Beau Biden, 40, the son of the vice president. Beau Biden recently returned from active duty in Iraq and has not yet made a decision to run.