To walk up and downstairs it can impact on their intellectual development also they have to think before taking them steps and how high the steps may be, the brain is connected to the motor skill development.
It can also impact social and emotional development, confidence links into this area, think of how big and daunting them steps may be to a little child so they might have to build up confidence to walk up or down them.
Here is a link with some useful info.
http://www.helium.com/items/1886506-...cal-cognitive-
Good luck!