Often as a child we are more capable to seeing different things from, spirits to visions, due to the fact that our mind is more open. Society has not yet told you that it isn't possible or molded your mind.
However, as you age and grow in society, your mind begins to block things out and your once open mind closes. Society begins to teach you "facts" and manipulating your way of thinking. It influences you through school, tv, and being around close minded people in general.
If you would like to change that, I suggest meditation focussing on opening your third eye.