I can't say I've ever been in a guild with a gender ban, in fact my current guild I'm building with friends for MOP has a female co-tank with our Warrior maintank. She's one of the best Feral Druids I've ever raided with, and very serious about her business. I remember being surprised to find out she was female the first time we spoke on vent all those years ago.
However, every guild I've ever been in has a hard and fast age rule. I agree on that whole-heartedly, there's a slew of issues that are perfectly valid from playing with kids that have nothing to do with personal maturity. From curfews, schedules of school to the potential for one of your raiders to just be literally grounded. Younger players just are a hard fit for a guild of adults wanting to make progression in their free time.
Add in the fact that a LOT of people under 18 online are vitriolic little shits, and it's a general no-no. I don't want Xbox Live getting in my WoW time, thanks.