What has 88 keys but cannot open a single door?

I'm tall when I'm young and short when I'm old. What am I?

I have a spine but no bones, pages but no writing on most of them when new. What am I?

I have a head and a tail but no body. What am I?

I have cities but no houses, forests but no trees, and rivers but no water. What am I?

I can travel around the world while staying in a corner. What am I?