i think the only reason why some people say you should get into modelling at a young age, because of the kind of lifestyle modelling has if you persue it as a career.
If you stick with something at a young age, you kind of adapt to that lifestyle of watching what you eat, exercise, and you stick with it.
sometimes its hard to get into agencies or get modeling positions, but if you start sooner your noticed more, you have experience, and you kind of have a feel of what agiencies are looking for.