Thoughts on parenting: When are my kids old enough for ______?

Some of the timing of raising kids is mapped out. For example, the consensus is that children should either be formula fed or breastfed until a year of age. Kids normally potty train between 2-3.5. School typically starts around age 5. Drivers licenses are handed out at 16, and, of course, "adulthood" is reached at age 18.
There are thousands of other milestones that aren't so mapped out for us, though. Over the Christmas holidays, I was having a discussion with some mothers in Indiana, and I was sharing that on Saturday mornings my two oldest kids have a routine. First, they know not to get out of their room until 8am. Most of the time they aren't up before 8, but if they are, D'arcy knows how to read the clock to tell what time it is. At 8, they are allowed to get up, but we don't get up with them. They make their own breakfast, and, no, I don't mean they eat cold cereal. They'll each make themselves a packet of instant oatmeal, and D'arcy will stick them in the microwave and cook them. Sometimes, they'll have individually packed yogurts, or peanut butter and jelly tortillas. How do they spread the peanut butter? With a blue plastic knife that they have permission to use. The other moms got a laugh out of how much my kids do or are allowed to do on their own. None of them had kids quite as old as D'arcy, but it made me realize how much of a judgment call each of these steps of independence can be for parents.
For example, at what age can a child safely play out front without supervision? We live on a fairly slow neighborhood street, and so, at the age of 5, D'arcy is allowed to play out front on her own. She has specific boundaries that she is not to cross which she is great about adhering to. She'll climb "her" tree or ride her scooter outside. The problem begins when her three year old brother wants to join her. Although he understands that the street is off limits, I don't trust him to be out there on his own. However, can he be out there with his sister?
I'm not looking for advice here so don't feel prompted to leave your opinion on this question. I'm simply sharing that one of the things that I find most difficult about parenting is deciding what level of independence my child is ready for. Two generations ago, when my dad was little, most kids were released outside in the morning and were told to be back in time for dinner. I was listening to someone on NPR express how nowadays kids have very little to no time that isn't supervised which from my observations is pretty accurate. A lot of children's play is in an organized setting; playdates, organized sports, etc. Which ultimately means that in two generations, we have greatly altered the way we parent.
I'm trying to be aware of this change as I make decisions about what my kids are ready for, and in general, I lean towards giving independence versus over sheltering. One of the qualities that I praise in my kids is being teachable. If a kid is asking for independence in an area, they have to be willing to receive basic instruction on the topic first. If they remain patient and teachable during the instruction, I think it displays, in many cases, a readiness for independence.