How early should you start to worry about a kid's weight? This study looked at medical histories of 111 fat kids (BMI >85th percentile). Their progression from normal weight to being fat started at about 3 months, and there was a "tipping point" at about 2 years. Half the kids were overweight by then and 90% by age 5.
It sounds like they're talking about a tipping point for the mass of kids rather than individual kids.
Their point is that doctors should review diets during early well-child visits, to keep them from getting fat rather than working on it after they're already overweight and fixed in their food habits. It keeps turning out that the answer to problems with adults is to start with them before they're 2 years old.