Always we consider the present age to be the base for finding out the past age or further age.
For example if the present age of a person is ‘Y’.
Age of the person after 10 year will be X+10.
Age of the person 10 year before would be x-10.
Present age of John is ‘25’.
Age of John after 10 year will be 25+10=35
Age of John Before 10 year would 25-10=15