Not to diminish aerobics, flexibility, and balance, but no one should ignore strength. The less muscle you have, the more pressure on tendons and ligaments. It helps build bone density. It helps with ordinary functions like picking up groceries, climbing stairs. I've seen older people struggle to get out of deep-cushioned chairs. Even walking on ice is easier if you have good musculature. And God forbid you fall. You want your muscles to take the brunt of the fall rather than just the bones, ligaments, and tendons.