I don't think that it would downgrade the forum any it might even pique interest .
I don't have a problem with a small fee or a donation to obtain a password. It would allow only people who really want to be in the area access.
The donation would be made with a CC and this would also help keep the younger people out of the area that they shouldn't be allowed into anyway. The donation would help defray expenses that keep the forum running.
A Beta v.1 Private Area could be done for a three month trial and then see how it goes.
Any other ideas?