I assume that guilds that want to incorporate a minimum age requirement do so based on the reasoning that while there are exceptions, older people tend to be more mature. I think that more often than not, they draw a line out of a desire to avoid doing a lot of screening.

Whether that's right or wrong, or fair or unfair, is a matter of opinion. In my experience, few guilds bother with an age limit, and some of those are not very strict about it. I actually wouldn't judge a guild's maturity level based on such a simple and easily-implied/applied metric. If I found a guild that was worth being in, and they had an age limit, that wouldn't bother me. A guild without an age limit that was populated by people 25 and older who acted like idiots wouldn't be a guild that I would want to join.

I've known lots of immature adults, and quite a few mature "pre-adults." But in general, older people are more stable and more mature.