It doesn’t matter how old you are. Twenty, 25, 30 who cares. All of us can lower our fitness age by staying right on top of it, taking good care of ourselves, especially physical activity.

on top of something为习语，意为in complete control of something.即完全控制某事。it指前面的fitness age, 即做体检时对某人的年龄的评估。例如一个80岁的经常锻炼的老人的身体测试，显示其各项指标相当于40岁的人，这个80岁老人的fitness age为40岁。因此fitness age可以翻译为查体年龄或体检年龄。stay on top of it=stay in control of it. 这里stay为系词后接介词短语作表语。意为保持对体检年龄的控制。