[Omranic]

Originally Posted by Alfa1

Almost the same error:I assume this is because I have set the memory_limit to 64mb in .htaccess, so that this is not reverted with server upgrades. I have temporarily set my memory limit to 128MB, then ran the cron without errors and then lowered it back to 64MB.

Sounds Good, that means it works fine with this solution.
A better solution may be available if possible, since the processing cycle already requires the same amount of overhead & it's approximately the same as default vBulletin ban process. The difference is the loop in which multiple users are to be processed respectively.

Originally Posted by Alfa1

But the 435 underage members are not banned by the cron.

What do you mean exactly!? Since the cron has been executed successfully, then it should do it's work!
Can you answer please on the following questions:

1. When you run the cronjob manually from AdminCP, what did you get? Did you get a report with banned users, or may be you get only a phrase saying "Done" without any listed users?
2. Do you've already underage users that aren't already banned before?
3. The "already banned" users, when there ban will be lifted?