Erm...you've just created a problem for me with this. I just went to send a message to someone about tokens that they wanted to buy and I can't send it. The trouble is that he has given me 0.1 ETH as a deposit so that I would keep a certain amount of them back for him and so that I could move some of it into the wallet where the tokens are so he could see that I really have the AGI tokens.
I just made it so that everyone who has sent a PM to a newbie in the last 30 days is already "opted in". Does that fix your issue?
Wouldn't it be better to allow all messages at first but give a warning with PMs from newbies such as something like "Find this message annoying? Go into personal settings to block messages from brand new/newbie accounts" etc?
I didn't do that because I think that only a certain subset of users (maybe a minority) want to ever receive PMs from newbies, so it seemed to make more sense for such people to opt in.