If you want the 20-25 year old group to grow in your local races, design the races around what they want. Regattas need to be well organized, in fun places, and have great parties after racing. Great parties mean, there must be girls there, not just the 50 year old guys talking about the good old days. This age group will grow themselves once the momentum starts if the scene is right. It must be girlfriend friendly, so yes, have real bathrooms, and for the ones that don't want to race, have a list of things to do while their boyfriends are out sailing. Shopping and beaches are typically big sellers, as well as other people to hang out with.
The college racing scene is a great place to look for crew, and when they graduate and are considering what boat to buy, cats will be an obvious choice.
There is not doubt that yacht club regattas are an obvious fit for this group. Larger events with other young sailors is a draw and make the scene more social. Its good exposure for the cat-class, and its my experience they are happy to have the excitement that high performance boats bring to the events. HPDO, BBR,and Newport regattas are good examples of this here in the northeast.
Lastly.... provide FREE BEER!