How to archive portions of Mail? (OS 10.4.11)

I have OS 10.4.11.

I'd like to know how to archive all the old mail in my mailboxes in Mac Mail.

I'd like to reduce my active mail.

But I want to keep the last 6 months-worth in the boxes.

I can see how I can just copy all the mail folders to my backup drive then empty out the boxes and start over. But I can't see how to back up just part of the mail. Easily.

Thanks! --JP

Hello? Anyone out there?

If no one is going to help with my question (about archiving Mac Mail, or portions thereof, conveniently), here's another question that someone might be able to answer:

Is there a Mac Forum where people do get help?

Thanks! --JP