Youth Leagues? Not that I'm aware of. Some sanctioning organizations have youth divisions, though. I'm not sure if they have youth divisions for MMA, or just kickboxing
Some of the sanctioning organizations you may want to check out are:
Global Combat Alliance (GCA):
http://gcafights.com
International Kickboxing Federation (IKF):
http://ikfkickboxing.com
United States KickBoxing Assoc. (USKBA):
http://uskba.com
United States Muay Thai Assoc. (USMTA):
http://usmta.com
World Kickboxing Assoc. (WKA):
http://wkausa.com
While many of these organizations seem to be kickboxing organizations at first glance, most have been promoting MMA bouts for years now.