Just wanted to clarify a few things.
You only need to be signed up with one of the organisations mentioned in the original post as they all work together to find a match. So there is no need to join another if you are already on.
The age ranges that are mentioned are just for signing up and you will stay on that register until you are 60. So please do not worry of you are over 30 you are still on the register. You haven't been taken off just because you are over 30.
The joining age range is to due to the fact that younger donors do provide much better transplant outcomes for patients