ROBERT HEVEY was fascinated by gardening as a child, but then he grew up and took a 30-year career detour. Mr. Hevey earned a master’s in business and became a certified public accountant, working for accounting firms and businesses ranging from manufacturing to enterprise software and corporate restructuring.

“I went to college and made the mistake of getting an M.B.A. and a C.P.A.,” he recalled with a laugh.

Now 61, Mr. Hevey is making up for lost time. He’s a second-year Ph.D. student in a plant biology and conservation program offered jointly by Northwestern University and the Chicago Botanic Garden. Mr. Hevey, whose work focuses on invasive species, started on his master’s at age 53, and he expects to finish his doctorate around five years from now, when he will be 66.

“When I walk into a classroom of 20-year-olds, I do raise the average age a bit,” he says.

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