My son started swimming at the age of two. He hates it. He cries before, during and after his swim class. But it was good for him because he's asthmatic. Fast forward to now, he seldom experience asthma attacks. His swimming everyday made him healthier and stronger. When he was 5 years old, he started playing soccer and basketball.
Aside from making kids healthier and stronger, playing sports is a growth booster. My son is growing up so fast, a year from now he'll be as tall as me. Start engaging your kids in sports while they are still young.