WASHINGTON — Edward Kennedy, Robert Byrd, Arlen Specter — fighters and history-makers all. Their battles with age and illness are the hallmarks of the nation’s oldest-ever Senate and reminders of the fragility of power.

The over-70 crowd is a caucus all its own, fond of self-deprecating humor and kindnesses that cross party lines. Ninety is the new 80, Mr. Byrd quipped recently.

There is no more forgiving place to age, as Senator Thurmond of South Carolina could attest. After retiring as the longest-serving senator in history, Thurmond died in 2003 at 100 years old.

Still, news of the 76-year-old Mr. Kennedy’s malignant brain tumor Tuesday was a heartbreaker even for this wizened group, which has seen spouses and friends fall before them.

Mr. Byrd, 90, wept as he prayed for “my dear, dear, dear friend, Ted Kennedy.”

“Ted, Ted, my dear friend, I love you and I miss you,” Mr. Byrd said from his wheelchair in only the second floor speech he’s given since a fall at his home in February. His wife, Erma, who died in 2006, “would want to say, ‘Thank God for you, Ted, thank God for you.'” The nine-term senator, now the longest-serving in American history, said.

Mr. Specter, 78, is balding from treatments for his second bout with cancer. Mr. Specter himself once received a diagnosis of brain cancer — and a grim prediction of six weeks to live. Despite his experience with the deadly disease, he told reporters that hearing the news about Mr. Kennedy was “just overwhelming.”

The fifth-term Pennsylvania Republican has said many times that staying on the job through treatment has been key to his survival. “If tenacity and willpower can do it, Ted Kennedy will be a survivor,” Mr. Specter said.

“I am so deeply saddened I have lost the words,” Senator Warner, a Republican of Virginia, 81, said.

Senators do not often find it hard to talk. Words are the currency they use to advance or kill legislation. This speed, born of experience, scholarship, and age, was part of the plan set out by the nation’s founders, according to Betty Koed, assistant Senate historian. The constitutional framers set the minimum age requirement for the Senate at 30 and for the House at 25. In the late 18th century, that was late middle age. Since the first session of Congress, in 1789, the average age of members of the Senate has risen to an all-time high today of 61.8 from 47, according to Senate records.