There are really two ways to address this question. The first is to ask, "Who is the oldest person to be sworn in as mayor?" The other is to query, "Who is the oldest person to actually hold the office?"
The answers are different because with the possible exception of Tom Murphy mayors continue aging after being sworn in. (Murphy always seemed ageless to the point that I suspected his executive secretary, Tom Cox, of being a kind of "deputy of Dorian Gray," becoming more dour every time Murphy went out for a jog.) So being the city's "oldest mayor" is a function of both your age when you took the job, and how long you held it. Even Ravenstahl could be the oldest mayor in Pittsburgh someday, should he preside over a decades-long era of peace and prosperity or at least redd-up streets.
When Murphy's predecessor, Sophie Masloff, left office at the age of 76, she was indeed the oldest mayor in Pittsburgh history. But when she took office in 1988, she was a mere slip of a lass, at a spry 70 years old. That makes her a spring chicken by Pittsburgh politico standards.
In fact, the oldest person ever to be sworn in as mayor was Frank J. Gallagher, who took office in 1959 at the ripe old age of 75. "Mayor Gallagher also lifts his title as the oldest mayor in the City's history," trumpeted the Pittsburgh Press account of his swearing-in ceremony.
In some ways, Gallagher was old even beyond his years. As a 1943 Press biography noted, "While still a child, [Gallagher] became an employee of the Oliver Iron & Steel Company and, at age 13, began a 30-year employment record with the United States Glass Company." His story was that of a lot of kids who grew up near the South Side's glass works in the 1880s: As columnist Gilbert Love later wrote, Gallagher's "first job was carrying blobs of molten glass from the furnace juggling and turning it en route to keep it from falling." No doubt such skills came in handy later on: In 1913, Gallagher helped organize a glass-workers' union, and later launched a career in politics, winning a city council post in the 1940s.
And Gallagher had something in common with both Masloff and Ravenstahl: None of them was originally elected to the mayor's office. All three ascended to the job from the city council presidency. Gallagher took power when his predecessor, legendary Pittsburgh powerbroker David L. Lawrence, was elevated to the governor's mansion. Masloff and Ravenstahl's deceased predecessors the beloved Richard Caligiuri and Bob O'Connor, respectively no doubt rose much higher.
Recently, there have been complaints about the city's rules of succession: Because of quirks in the city charter, Ravenstahl could still be mayor through 2009, even though he was elected only by voters in one council district. Plus, it's clear that council presidents are chosen with little thought for whether they'd be good mayors. Back in Masloff's council days, for example, one of her critics, Michelle Madoff, told the Post-Gazette that while Sophie "doesn't do anything I'm going to recommend her for council president. It's not a big deal."
On the other hand, Masloff, Ravenstahl and Gallagher brought some added diversity to the mayor's office, representation we might not have gotten through a more typical election process. Ravenstahl was the youngest mayor; Masloff was both the only Jewish mayor and the only female one. As for Gallagher, his staunch labor background newspaper columnists lauded his "crusading zeal" and noted his past efforts to investigate sweatshops and company towns might have inspired corporate opposition if he'd campaigned for mayor.
But he never did. Instead, he made clear from the outset that he would serve as mayor for only a year, and then return to council. A more lasting successor, Lawrence ally Joseph Barr, won an election held in 1959, and took office in December of that year. Masloff, meanwhile, ran for reelection a year after taking office, and served through 1992. Ravenstahl seems likely to run whenever we figure out when to have an election.
But when left to their own devices, voters usually seem to prefer the leadership of Irish males. If you want to bring some diversity to the city's political leadership, it seems, you have to do it the hard way.