UBB.threads does not have a tool to prune Private Topics older than a specific date; the closest tool to that would be the Prune Topics tool, which doesn't touch PTs.
UBB.threads has three tables for Private Topics, ubbt_PRIVATE_MESSAGE_TOPICS holds the topic information (the topic id, the timestamp it was created, total replies, subject, creation user, last reply time, topic poster id, and last poster name), ubbt_PRIVATE_MESSAGE_POSTS holds each individual post, and ubbt_PRIVATE_MESSAGE_USERS holds information for each user who is involved in the private topic.
With this data we can extrapolate that data will need to be purged from three tables in order to prune all of the data. You would have to create a PHP script, or custom query, to check the timestamp for TOPIC_TIME in the topics table, you'll need to check if that timestamp is equal to or greater than the current time (as a timestamp) minus however many years you wish to prune, to build a list of topics.
Now that we know the Topic IDs of the topics that need to be pruned, we'd take the TOPIC_ID and then purge the data from the other two tables with entries that match the TOPIC_ID.
Another option would be to just truncate all three tables (empty them) and have your users start from 0 PTs.
