Admittedly, I never had much of a personal interest in pokémon performing or showcases in general. This is more Lanette’s field, to be quite frank, so I’ll allow her to take it from here.
LH: Well, to start things off, most regions don’t hold pokémon showcases, largely due to the popularity of pokémon contests, a similar sport. Just about the only places where you can participate in showcases are in Kalos (and Europe in general) and Unova (although musicals are more popular over there). Still, it’s worth it. I personally enjoy watching showcases more than contests, just because there’s more variety in the sub events and emphasis on style and synchronization between pokémon and performer.
On that note, practically any pokémon can participate in a showcase, but it’s important to know that, just like in a contest, you have to be strategic about who you use. Of course, most performers don’t know the theme of the first half of a showcase until it happens, so unlike in contests or even gym battles, I don’t mean research the next showcase when I say be strategic. What I mean instead is, of course, choose pokémon who work well together and whose abilities can be combined to highlight not only each other’s skills but also your creativity. Of course, building a routine for the second half of a showcase (the freestyle performance) is important too, so always keep that in mind when choosing which pokémon to enter.
As for your age, that’s rather simple. Just like with contests, you can’t enter showcases unless you have a valid trainer’s license. In most regions, this means you must be ten or older in order to register.
The Pokédex According to Bill 