Answer:
I belong to you but others use me more often than you do
I am a box which holds keys but not locks. With the right combination, I may unlock your soul. What am I?
I am owned by the poor, the rich do not need me. If you eat me, you will die! What am I?
I'm tall when I'm young and short when I'm old. What am I?
What goes up and down but does not move?
No matter how little or how much you use me, you change me every month.