Just a general comment after reading all the posts thus far.
If someone asked me if 7 was too young to turn a kid loose in UO without any supervision, I would (after giving them my best "are you EFF'ing crazy??" look) tell them that UO is not the place for a 7 year old to be running around on their own. I fully agree with anyone who feels this way. HOWEVER.....
Children NEED to be exposed to life, they need to get hurt, feel emotional pain, run until they're out of breath, be told "no", experience adversity, taught discipline, know what responsibility is, experience love, endure social awkwardness, overcome challenges, and above all, they need a parent to be there to help/guide them through all of this and to teach them the difference between right and wrong. Children are all different. Some need more guidance and hand-holding than others, but one they thing don't need is a parent who shelters them from everything. Just like Black Rain and others have said, this kind of thing does not end well. Having joined the Army when I was 18, it was astounding to me how many "kids" go to basic training with little to no life experience. My job as a parent is to prepare my children for adult life - period. Along the way I hope they have a wonderful childhood, but part of that journey includes learning about relationships between cause and effect, actions and consequences, and learning how to make good decisions based on the information available. I see UO as a good way to help share time with your child that can also have some very positive influences on them. I see it no different than any tool, all of which have the capacity to create wonderful things, or destroy. A saw and hammer can be used to build a home for a needy family, or kill someone and dispose of the evidence. What you do with your child's brain and how you use the tools available to you makes all the difference in the world.
Go ahead, feel free to nitpick my post and disagree with me. I know my kids, now 15 and 12, are very intelligent, well-rounded, highly mature, and (according to their teachers) bizarrely honest children. My daughter (15) is in JROTC and is planning for a career in the Air Force once she gets her nursing degree. My son is nearly 13, but he too has his sights set on the future as some kind of scientist, probably in the geology or volcanology field. I may not be the perfect parent 100% of the time, but with the constant congratulatory comments my wife and I hear about our children from people we know, teachers, and strangers when out having dinner, I believe we are doing at least one or two things right.
$.02