Hi Mark,
There is a way to change a member's birthday, but it involves going into your member files via FTP. Here's how to do it:
1. In your UBB.classic control panel, go to Member Management and search for the user who is having the login problem.
2. Take note of his member number (right after his user name in the search results).
3. Access your board via FTP, then go into cgi-bin/ubbcgi/Members.
4. Find the user by his number (should look something like 00000001.cgi if 1 were his user number).
5. Right click on that file and choose "Edit".
6. The code for that member's profile will come up - find the line where his birthday is listed (should look something like 20010101|yes(or no) if his birthday is listed as January 1, 2001). The next line will show his IP address.
7. Change his 20010101 birthday to his actual birthday (or anything older than 13) and save the file (File > Save).
8. Close the file and click on "OK" to re-upload the edited file.
9. Ask the user to clear out any cookies from your site - he should be able to log in now.
Hope that helps!