Dont worry about your age and Pony Club Tests, many of the show venues that run Walk & Trot tests use the Pony Club one but are open to anyone, whatever age you are. As some one else said they are a good introduction for a novice be it horse or rider.
Hope you find a venue running a walk/trot test and feel up to having a go, you will be fine, good luck and enjoy.
Thank you MrsT. Oliver is 6 and I've no idea if he's ever done a dressage test as I don't know his history. I am definitely a novice myself though and will start looking for somewhere that runs a walk/trot test because I want to start with something simple and well within my capabilities and then build up to something bigger if I enjoy the first experience.
I started off doing the walk & trot test at my local equestrian centre - I actually enjoyed it ! Plus, they normally put you on first so there's even less people watching than usual.
Solihull RC usually run walk and trot tests. I think you can download the tests from their web site, so even if its a bit far to go, you can still have a look and see whats involved.