The CTIME column of SYS.USER$ is the date and time the user was created. So this value does not help you.
I know of no way to determine the actual date and time the password was last changed. However, you can set up a profile for your users to expire passwords after a certain number of days. The EXPIRY_DATE column of DBA_USERS will show you when the password will expire next. To determine when the user changed their password last, simply subtract the password lifetime value from the expiration date.