There's a tremendous difference between physical fitness and running your body into the ground through long days of extraordinarily difficult labor.

I suspect acrobats self select as people who look young, as that seems correlated with agility. Not withstanding that, the most common advice my high school coach gave us was to not over-practice. (I ran cross country, most of the more dedicated members had at least 1 season where they had to drop out from over-exerting themselves).