How old is old?

There are the familiar clichés: "Age is just a number." "You're only as old as you feel." "[Fill in the blank with an age] is the new [fill in the blank with an age 20 years younger]."

There are the clever quips from writers and celebrities, like Maurice Chevalier's "Old age isn't so bad when you consider the alternative."

They're funny, pithy, wise. But also, let's face it, a bit shopworn.

Like anything else, adages about aging can use some freshening from time to time. So we asked a few local aging experts to answer the question: "Just how old is old, anyway?"

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Catherine Sullivan said she had just been thinking about this question. That's not entirely surprising, given that Sullivan, 53, is president of Minnesota Gerontological Society and teaches a course on aging at St. Catherine University. Feeling old, in her view, is about letting others define your limits.

"People will say, you should do those things or you shouldn't do those things based on your age," she said. "If people internalize that, they become old."

Lisa Edstrom just turned 50, but already knows what Sullivan is talking about. A geriatrician recently advised Edstrom, an associate director at the University of Minnesota's School of Public Health, to curtail her hobby of bike racing.

Most days, though, what constitutes "old" is "a moving target," Edstrom said. She has friends her age with kids at home; others with grandchildren. As a current Ph.D. student, Edstrom enjoys the advantages of having acquired more life experience than the average student, along with the disadvantages of taking a graduate-level statistics course when her high school math classes are but a dim memory.

"Some days I'm in a situation or a context where I feel like I'm older, and some days I'm in a situation or context where I don't feel old at all," she said.

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But at 79, Jan Hively said she's begun to get a sense of life's finite quality. Hively, who was 69 when she earned a Ph.D. focusing on "productive aging," co-founded three organizations to help keep aging people active, creative and, well, productive. "Meaningful work, paid or unpaid, through the last breath" is her mantra, she said, and one she clearly pursues in her own life.

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If Robert Kane is probing connections, it's those between his present and his past. Kane, 71, who holds the Minnesota chair in Long-Term Care and Aging at the University of Minnesota's School of Public Health, describes old age rather poetically as "the strange face in the mirror who reminds you of a younger self."

But does he consider himself old?

"Old." said Kane, "is 10 years older than the people around you."