Look closely at the way the numbers are distributed.
People get heavier as they get older.
In this first example, the only effect of age is to produce a uniform increase in weight, irrespective of height. It's just as correct to say there is a uniform increase in weight with height, irrespective of age. These interpretations come straight from the model. Or you can look at the graphical interpretation and think about the effect of age as altering the intercept of the weight-height line in a uniform way. In the example, I've given older people a greater weight for a given height than younger people, which is not necessarily realistic.