How do you feel about it?
I think at my barn, which is smaller, it's more of a case-by-case basis. I feel it's a good rule for minors and beginners. If it's an adult who doesn't train with the BO (who is also the trainer at my barn), and has their own horse, I think it's a little too much in their business, unless it's an absolutely dangerous situation. If it's an adult who does not, I definitely think approval should be necessary.
Do you feel it's more for the rider or for the horse?
I think it really could go both ways. Sometimes it's one way around, sometimes it's the other.
When do you feel a horse is ready to jump?
When they are balanced, responsive, and comfortable at a w/t/c with a rider, and after they have been trotting/cantering ground poles and cavaletti to the point they are balanced and comfortable over them.
When do you feel a rider is ready to jump?]
Much like the horses - when they are balanced and comfortable (and in control) at a w/t/c and over ground poles and small cavaletti.
Do you feel that this rule has been enforced and is effective?
I feel like my BO enforces it well enough. I don't think there's a clear written in stone rule, but I think she uses good judgment on a case-by-case basis.