Promotions don't seem to work.

[kh99]

You can only promote on posts >= the value you give ("Reputation Comparison Type" only applies to strategies that include Reputation). So you could create a user group with no PM permissions and use "Prune/Move Users" to move all users with < 15 posts that new group, then add a promotion to Registered User with strategy set to "Posts" and "Posts" set to 15.

Of course you could also do it the other way - disable PMs in the current group and make a new one that is allowed to PM, then you don't have to move any users.