I attend mediumship classes and medium/spirit circles several times a month. So far I have only been able to tap into people's past lives. There are personal friends of mine in the class that are able to read objects owned by others. That is part of our training. I haven't been able to read one darn thing.
The medium that teaches our class says that everyone has the ability to become a medium. She says it takes direction, devotion, and a will/desire to be able to channel spirits. I find I am fearful (not sure exactly what I am afraid of), and that prevents me from letting the spirits through.
I do believe some people are natural born mediums. Several of the mediums I know personally say they have had the gift since they were young children.
Just a note... I took a picture of my sister to class in a sealed evelope. Without opening the envelope, one lady told me where my sister lived, how old she was, what she did for a living, what color and style her hair was, and what she was wearing in the picture. Blew me away.