I remember 18 months being a really key time for stacking it, I think my girls had been walking for a bit but by 18 months they had got more confidence and picked up speed and splat! I think it is just a part of life when they are still learning the skills and growing lots. And to a certain degree the human body is pretty well designed for this stage. Every child goes through it, some more than others though.......DD2 I'm talking about you!