It's because when you're young you have more free-time and often higher test which does promote muscle growth. So you naturally live a more active lifestyle and (while still growing) use more calories. But if your habits are shitty then it's just a matter of time whether you'll be fat at 5, 15 or 25. If you have good habits you'll stay lean the entire time. TLDR: Natural growth of body can make up for shitty habits while still young.