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The /etc/shadow file defines for each user a number of password aging related parameters. See man shadow for the structure and interpretation of these fields. To display the values of these password aging fields in a human readable format, use the command chage -l <username>. You can use the same command to modify the values of those fields or the usermod command (however the latter has less options for changing password aging fields). Check out the /etc/login.defs file for the PASS_* config parameters and the /etc/default/useradd file that define a few defaults for password aging fields. Password aging is described in more detail on this page.