They mean that if you do overwhelm your metabolism or fail to learn motor skills as a child, you may be ruined for life.

I know some people who just hate exercise. I think they has no habit of movement of any sort. Likewise, people who were fat or weak as a child have no baseline sense of what it feels to be healthy - and neither do their bodies cells.

So kids can handle more sugar, but if messed up the consequences are worse.