I think everyone's getting a little carried away with the age limit thing. Yes, sometimes the little kids at Vanguard can suck. However, especially for a field just starting up (if this actually came to fruition), they're gonna need all the beginning revenue they can get, no matter who it comes from. I say start off completely open to anyone, and then as things settle down and you start to see more of a regular customer base on a fluid basis, then you can start implementing things like "adults night only" and that kind of thing. You don't want to instantly cut off a potentially large section of your customer base right off the bat though... not good for business.
Keep in mind, this is coming from the viewpoint of an adult. I am still a complete supporter of having an 'adults only' type of deal at a field, but not 100% of the time, and certainly not right as the field opens. I'd rather deal with kids some of the time than have the place close down.