At my Tang Soo Do school, we have a Black Belt Club.
The instructors will usually note if someone is very motivated and then ask them if they would like to join BBC. For kids, they usually have to be at least an orange belt.
For adults, it can be as early as white belt. My daughter joined when she was white belt. I was approached about it but am not going to join because the mental overload would be too much. I've only been attending three weeks and I am motivated, I know where I want to go, but I couldn't handle the mental aspects of BBC.
There are pros! My daughter can attend classes as often as she likes, not just twice a week (original contracts are attendance once a week or twice a week). So, she goes three times a week which is fantastic. BBC really pushes the student mentally and physically. I think it's great. My daughter has learned Chil-Sun Iriyan (sp?) and could perform it at a competition if she liked. She's currently working on another black belt form. She gets to attend a board breaking clinic coming up in June, another perk. She's learning to do shoulder rolls, cartwheels, one-hand stands, and so on - I'm not sure how they apply later on.
Pros for the payee is that the tuition is locked in for the next 3 - 3-1/2 years. It actually costs less to be BBC.
For the motivated student, I think it's wonderful, if they can handle the mental aspects. A downside might be that during black belt club week, the student isn't working on much of the material for their current level. Then again, if they are attending two or more times a week and are motivated, they should have no problem learning their material, right? Another downside is that Master might ask BBC members to perform the black belt forms during a test or spotlight. One could fail if they're in BBC and can't perform the higher level forms. Therefore, someone testing for their yellow or orange belt (beginners!) could be asked to do Chil-Sun Iryan. I don't know if that's really a downside

. I suppose it depends on how comfortable one is with the forms.
I believe the more forms one learns, the more flexible they will become and the more balance they will gain. I'm sure there are other pluses I haven't even thought of. Wouldn't students be better at sparring as well? I have heard it said that those who are best at sparring are excellent at forms.
Personally, I know I'm not ready. I haven't even learned to square my shoulders correctly during Gi Cho Hyung Il Bu/E Bu, my front stance is more like a high-wire act most of the time, and I forget to bring my fist back to my ribs after punching. So, can you imagine me trying to learn the Chinese 6?-star form???? No way.
I admit the board breaking clinic in June is a tempation. But there will be other board breaking clinics when I'm mentally ready.