From Time magazine:
Modern exercise science shows that working with weights—whether that weight is a light dumbbell or your own body—may be the best exercise for lifelong physical function and fitness.
… [T]he “load” that this form of training puts on bones and their supporting muscles, tendons and ligaments is probably a bigger deal when it comes to health and physical function. … Through a process known as bone remodeling, strength training stimulates the development of bone osteoblasts: cells that build bones back up. While you can achieve some of these bone benefits through aerobic exercise, especially in your lower body, resistance training is really the best way to maintain and enhance total-body bone strength.