Well here's my 5 sec crew course...free of charge. BTW I got my first crew job mid thirties, put the age thing out of your mind because you're the only one who cares about it.
On the day, be relaxed as you can. Do not care about the outcome of the day. We've all been told no thanks at the first elimination round and we've also all been told yes at some point too. It's part of getting there, so enjoy a rejection if it happens because you are one step closer to the yes.
Listen to others, chime in when appropriate and be honest and natural in your reactions. Recruiters have seen it all before, they know when you're acting and know when you're genuine. So be genuine as much as possible. Airlines do not recruit all the same personality. They will typically pick some outgoing ones, so reserved and polite ones and some assertive ones. It's about having a mix of people who all display some common traits.
Now as for those common traits....
Are you respectful and polite at all times?
Do you wait easily and patiently?
Do you talk positively about yourself and others?
Do you present well and how does your presentation wear over the course of a day?
Do you interact with lots of other people well?
Do you pick things up easily and do you pay attention?
Can you think on your feet and come up with solutions outside of the box?
Know beyond all doubt that you are crew. Not that you will get a crew job one day or if you are very good your dream will come true. Think, act and be crew right now before you have the job. It makes a HUGE difference to the outcome. I used to work in cafes and everyday behind the counter I would think and act as if I were working on an aircraft. And yes this will sound insane but everytime I got the dirty dish cart out to push around I'd pretend it was a service cart. No-one else knew what I was doing but it I scored crew job very quickly after that because I never doubted for a moment I would be hired. That calm confidence really came off at the recruitment day.
Good luck. You will still get something out of your course day anyway so don't worry.