Resistance training is only good for you if you’re under 30, and really bad for you if you’re over 40, give or take. Cardio (running, swimming, stairs) is good for you no matter what age you’re at. The reason is you’re working your cardiovascular system with volume work, basically training your heart to pump more blood per beat, but with resistance training (pressure work), everything is contracting (and raising blood pressure) to move some heavy weights, instead of moving your own two feet, or if you’re swimming and you’re buoyant. Light and moderate resistance training is OK if all you’re trying to do is staving off brittle bones as you age.