It's not just about maturity (to be honest we can be as childish as the next bunch ) One of the main factors is shared dialogue and interests. To be honest most younger players woudl find our chatter dull and woudl not share any common ground which tends to be the case after a certain age.
I agree , some kids in the game are great ! and some adults can be right idiots - but in terms of a shared frame of reference the over 25 thing works quite well.
It is not that we hold anything against the young folk in general, if fact i did hear a rumor that one or two of the guild used to be children. Not Ulloh^^ though, i am not sure he ever was. In fact many members keep children as pets i believe.
However ensuring that everyone is an adult means that no-one can put them self in a difficult position by saying inappropriate things. (like groovy for example)
Also being a bit older gives you a change on identifying some of the karaoke often herd over TS in raids (only a small chance, if you are still in your twenties however)
As a member of The Garrison you may find it hard to fit in if you are a bit young. It's the "Tone of Voice" we use (is that the right way to say it) :P
Also, even if we have some really Hardcore players we are a casual guild. Young Hardcore players should seek out to other hardcore guilds to get the full gaming experience.
The 25 year rule is one of maney rules that keep The Garrison together as a Guild. If you are planning to start a guild of your own. You also should make a simple set of rules that will be the baseline of what you accept and what you don't some will accept those rules others will not. It will always be like that!
Mature children...
lol 
