Well, there's definately a problem if you have minors coming to your game. After all, if you are the host and some of your players are too young, you have to be responsible for them and that's a lot of work.
Here's something to share:
This issue came up for me when these bunch of kids were looking for a group to play with. My group's host didn't want anything to do with them for the reasons above. So I GMed them instead. I'm only 20 so I still remember how to GM. Also, I considered it a challenge to GM for younger groups and cater to different gaming styles (and maybe convert them to my fave style).
I sometimes don't like it when people require a minimum age for their players (though its entirely their opinion and choice, and I respect that). I often avoid those groups that call for gamers with a minimum age.
I started gaming with those older than me. Some of my best gaming friends were 5-20 years older than I was. I was the youngest of the group as well. I know what it feels like to be the kid of the bunch. But despite my age, they respected me and treated me like an adult. I was kind of the exception to the rule concerning age and maturity (at least, that what they said).
Since then, I've learned to take gamer's ages with a grain of salt. I usually consider each person's maturity individually, regardless of his or her age.