When we lived on base the rule was they had to be 10 to be left alone and 12 to care for a sibling. I started working outside the home around that time. I worked on base so I wasn't too far and they weren't alone long. Dd has taken the Red Cross babysitting class so she knows CPR and First Aid. They have rules that they have to follow and I know from the stay at home moms in this neighborhood and our last neighborhood that they actually do follow the rules! (At least for now.) I would not leave my 11 yr old in charge of a younger sibling. I didn't leave mine alone until I would trust them each on their own. Dd never "babysits", "watches" or is "in charge" when we are gone. Ds is old enough to be left alone and I don't want either of them to feel like they are the one taking care of the other. They are responsible for themselves.
"God put us here on this carnival ride, we close our eyes never knowing where it will take us next"
Wheel of the World by Carrie Underwood