By changing this setting, will this automatically purge the body from all older messages?

It will delete the message on the server, and hence also in Thunderbird.

Now say I view a message from 2 years ago and it downloads the body.

There won't be any messages from 2 years ago anymore.

Thanks for the response. I'm actually not looking to delete any messages. I'm referring to the Disk Space settings under Synchronization and Storage. Under there is an option "Synchronize the most recent 'x' days." If I enable this option should it automatically purge the message body of anything older than the chosen number of days for the local copy of the message?

From my testing it doesn't seem to do this. I have done a compact after enabling this setting but the file size of the local Thunderbird profile doesn't decrease in size. However, if I create a new account with this setting enabled from the start, the profile size is very small.

I see. I don't think the "Synchronize the most recent 'x' days" option will free up space for messages already downloaded or 'synchronized' respectively.
So this would only be useful if you delete both, Inbox and Inbox.msf files of your IMAP account when Thunderbird is closed. Upon restart, it would only download the body of messages from the most recent 'x' days.
As a result you'd free up space previously occupied by older messages, but it's a bit tedious. And local disk space shouldn't be a problem anymore these days.
So what exactly are you trying to achieve?

Thanks, that answered my question. Disk space certainly isn't an issue and the main reason I am looking into this is for roaming profiles. Thunderbird has always worked great no matter how many messages or size but when you have to download 5GB+ over the network for a roaming profile, that can be a slow login.

When your question has been answered, please mark the thread as 'Solved'.
Thank you.