The part of the mind used for spirit communication is the imagination, so I would say that this is why children are more open to spirit as they are much more free with the use of their imagination.
In the centre of the brain is a gland which scientists have discovered is actually very similar to an eye as it contains the same rods and cones as an eye.
This seems to show that when mystics called this the third eye they actually knew alot more than people give them credit for.
This gland is called the pineal gland and I believe is our "mind's eye" and link to the spirit dimensions. Sadly modern society frowns upon using the imagination and we condition children to detach from their link to spirit.
This would explain why only children have imaginary friends.
Unless you are a little bit strange like me ;) lol
Hope this helps