Field botanists are those who tromp back and forth observing, classifying, and preserving plant life, so of course it would behoove them to stay in shape. Bad knees, aching backs, failing eyesight may spell the end to outdoor life.
Botanists who've opted for research and teaching jobs have a good chance of hanging around untill retirement. Tenure works wonders in the academic world. Pharmaceutical companies are squeezing out more potential products from the plant world, keeping their industrious scientists very, very busy.