Quoting
aardvarc
there's not a flat number or age. If any complaint is made, authorities will look at the maturity levels of the children involved, the overall safety of the home, and related factors. Twelve is often considered an unspoken appropriate age to be babysitting, and would be considered in light of the number and age of the other children involved (better to care for older children who can talk, as opposed to infants, etc.). Then again, there are some 17 year olds who have no business caring for even one child of any age. In other words, each case generally gets evaluated individually. I've taken 911 calls from children as young as 10 who were calm, cool, and collected in an emergency and saved someone else's life - as well as calls from 38 year old adults who totally fell apart - so levels and abilities can really vary. If you feel comfortable that your child is mature enough to watch your other children, most of the time the legal system will follow your feel (since you know your child best) unless there is some glaring evidence to the contrary.
Some things you can do to improve the skills and abilities of your eldest to watch other children (yours or anyone else's) could include:
1. Call your local fire or health department - there are cpr and first aid classes geared to all ages, and most are free for attendees under 18 (can never have too much training on this, at any age)
2. Make sure you have an easy to use list of emergency numbers kept in a constant location in your home (inside the cabinet by the kitchen phone is my recommendation) and that all of the children in the home know where it is and what numbers to call for what (remember, if something happens to your eldest, one of the other children might need to seek help) - and make sure there is a basic first aid kit (especially include ice packs - either the crush-and-use kind or some in your freezer - kids often freak out when they see swelling)
3. Make sure all of the children know your full name (children often panic and can only tell people about "mommy"), where you work (name of the business, the address, your boss's name, and business phone number) as well as their own address and some basic directions on how to get to your home (major streets)
4. Make sure your children know any trusted neighbors, their names, and how to contact them if they need help - also make sure the neighbors have your contact info (work and cell numbers) in case your children need to vacate your home and show up there - you don't want them running back into a burning house to try to retrieve the contact list in the kitchen
5. Make sure your eldest is comfortable calling 911 - often children who have "played" on 911 in the past and had an officer show up and they got in trouble will never call again, even when they should; at the other end, you want to encourage your child to call 911 if they run into something they're not sure how to handle (something scares them, someone strange knocks on your door, obviously if someone gets hurt somehow, etc.). Make sure they know that it is ok to call 911 for things, even if there's not blood and guts everywhere (children are often resistant and don't think of calling unless something really big happens - they are used to adults calling for help and even 16 year olds can "freeze" and sit there wondering who is going to call for help - make sure you explain that even just being scared is a good enough reason). Police (and you) would rather be safe than sorry.
6. Make sure there are fresh batteries in your smoke detectors and that they work, then set it off so your child will be familiar with the sound, and show them how to clear or reset it
7. Have "the talk" about strangers, whether its the mailman, the meter reader, the nice guy looking for his lost puppy, etc. The door opens for no one. Ever. Even if someone saying they are the police show up without being called, teach your kids to call 911 to verify that it's the police before they open the door. Believe me, the police would rather you do this.
8. Make sure there are plenty of 'no cook' or microwaveable food and snacks around - you don't want them cooking without you home unless they've got a lot of prior experience
9. If you don't have a regular wired phone (you use cell only) make sure there is either an alternate phone or an extra charged battery around - teenage girls can talk much longer than a phone can hold a charge, and you want to make sure they always have a way to call you or help if needed and haven't talked the phone into the proverbial ground
10. Remove any firearms from the home, or keep them in a place separate from the ammunition and under lock and key - i could tell lots of horror stories, but they all have one thing in common - parents who thought their children either didn't know the gun was in the house, or if they did, that they had no interest in it or had already been warned to stay away from it. Take no chances. Gun. Ammo. Never together with kids in the house.
Covering things like these not only makes your kids safer, but if there should ever be a question about the abilities of who is watching the children, having made such preparations and discussions ahead of time can really cya.