Not to seem victimized here, but being a young gamer (13 years old), I think I can answer a few of these questions here.
For the first, I find that limiting content based on age is a sheltering process. If children don't see the mature concepts of these at some point during their early age, they will take certain concepts as a shock when they face reality in their later years. While many people blame video games for the violence in our youth, they shy off on the concepts of multiple reasons why youth can be violent; Vulgar Music, Terrible Education, Graphic Movies, Emotional Neglect for Parents, and Peer Pressure all contribute to the violent ideals of today's youth.
The reasons why children are disliked on most online services is due to their inexperience and impoliteness in a public based situation, and because identities are in the basis anonymous, they feel their actions don't carry and consequences or ramifications. Their inexperiece come from manners; most of our youth nowadays aren't taught certain concepts from their parents to deal with public occupations due to their parents sheltering them from reality. certain concepts not taught are: being quiet in public, being polite in public, and learning how to deal with other people's opinions in public. Frankly, the mic isn't a freakin' slot box for god sake.
While hours of gaming can be harmful at a young age, taking time away from gameplay sessions can help the young user physically and socially. For example, my gaming sessions are 1 to 3 hours depending on the time I have on my hands. Outside of that, I hang out with most of my friends and usually head to the mall to pick up some chipotle or chick fil a with them. Physically, I have always been a rather small person, but as i grow through puberty, I have learned to exercise daily in 1 to 2 hour periods. Also, despite my gaming tendencies, I am able to complete school assignments with ease and can work in a timewise manner. Even out of my gaming hobby, I have interests in music, improvisation, and graphic novels. While younger kids may be affected at their age, as they go through puberty, they will probably begin to understand the benefits of supporting social and physical health, doing well in academics (that is, if their parents support them), and learning to offset time for other interests.
While I might be getting a bit preachy about this, I hope my opinions are set.