You can't keep them in their old classes when they turn 18, because MLS has automatic rules that move them to what it thinks should be their new class. But that's not an insurmountable problem. You just need to manually move the members from the new class back to the old class if you still want them to be in the old class. It's the same issue for YW who are automatically moved to RS.
To manually move them, the easiest way I've found is to just remove them from the new class or organization, and then you'll be prompted for what the new organization should be; specify the previous organization and they'll be moved back.
Questions that can benefit the larger community should be asked in a public forum, not a private message.
