How Old Is Old Enough To Play Outside Alone?

Our new neighborhood is fabulous, tons of young families, and tons of kids. I love that my kids have friends to play with and can safely ride their bikes out front and play in our grass with no worries. My kids are 11, 10, and 8, and they are allowed to play on our street (not a through street) but not allowed to cross the streets on either side or play inside anyone's house without asking permission first. I tend to worry more than I need to and my 8 year is less mature than the older two so I don't even let her play outside unless her older brother and sister are outside or at least one of her friends (you know safety in numbers).

In our old neighborhood in Maryland their were lots of kids that had much further boundaries than my kids, like crossing major streets and going alone to McDonald's or 7-11. I quickly realized my kids weren't going to have the same boundaries as the other kids and I was okay with that. In Maryland we lived on a dead end street with no traffic and my neighbor and friend would let her 4 year old roam around her backyard and my front yard with his older sisters, which seemed okay given their was no traffic and always a ton of older kids outside.

Here in our new neighborhood there seems to be two types of parents, which seems to correspond with both their parenting style and the age of their oldest child. The first type of parents tend to be more conservative: they know where their kids are at all times, they are outside with their kids, they know the other parents in the neighborhood. The second type of parents are a lot more relaxed: they let their kids roam around the neighborhood, the neighborhood park, the lake, inside people's houses they do not know.

I guess I fall somewhere in the middle since my kids are a little older: I always know where they are but I don't have to be outside with them at all times, I've tried to meet as many of the neighbors as possible, especially the neighbors with kids, and I only let my kids play inside one of my neighbor's house, who I've gotten to know over the past month. The neighbor kids keep begging my kids to ask to have a sleepover but I've told me kids that won't be happening until I know their families a lot better.

Our neighborhood has a couple four year olds from different families that roam around minimally supervised. Yesterday one of the little girls was riding her bike outside alone and totally face planted several houses down from her home. No one was outside besides me (cleaning my garage) so I of course ran over and scooped the girl up. I barely know the kid so after picking her and her bike up she wanted to ride home, screaming, alone. I don't blame her, she doesn't know me from any other stranger but where were the parents or at least the older brother or sister? Today a different 4 year old spent a lot of time at my house without probably anyone knowing she was there. She wasn't hurting anything but it just amazes me that 4 year olds roam the neighborhood without supervision. Call me paranoid but there is no way my 4 year old would be outside alone. I know the chance some scary person would scoop them up off the street is almost slim to none but what about traffic or getting hurt or just getting into trouble. I would just worry myself sick and really it isn't worth it.

So what do you all think? Am I paranoid? Should I be more relaxed? How do you handle this with your children?