This book is  older than  that one

and

I am  elder   to  him

Tell the difference why there is not 'than' use in second sentence????????

Here are meanings of both the words. Hope that help you understand the difference between the two:

• Older: This means relatively advanced in time or age. It is having lived or existed for a relatively longer period of time.
• Elder: elder is an expression which means ‘older than you’

Here are instances of sentences with their usage which would make the concept clearer to you.

• He is older than his sister.
• He is elder to his sister.
• The monument is older than Red Fort in Delhi.
• The king was elder to his seven brothers.