I don't think that's always possible. Log files on Linux systems are rotated, and typically only kept for a few weeks, or months at most.
The only thing you strictly need to do to add a user to a system is to add a line in the /etc/passwd file. This would not log anything beyond the changing the last update date of the file each time you add a user, so you can tell the last time a user was added (or any of their login details changed), using:
More commonly, the useradd
script is used, which will create a home directory for the user. Some filesystems store the directory creation date, but most don't so you probably can't figure it out this way.
I just tried adding a user to Ubuntu, and it didn't log anything. There are a few patches around that you can use to modify adduser to get it to log the creation of new users; you might try looking for a file called /var/log/adduser.log or similar.
You can run the command lastlog
to find out the last time each user logged in, and where they logged in from.
If you are using sudo to create users then all sudo commands get logged, usually into /var/log/secure or /var/log/auth.log
Hope that's of some help!
—Robert J Lee