Your age isn't printed on your student card (one of the reasons you can't use it as ID at outside clubs).
You will always be fine getting into KCLSU places with your student card though, that's taken as enough proof that you are overage.
I'm not sure what's happening about events before people have registered. It was never a problem for me cause my registration was on the Monday. I've got training tomorrow, though, so I'll ask the management and get back to you.