Two common questions are at what age can one child watch a younger sibling and also at what age can my child be left alone. In New York the answer is a very clear non-answer..."it depends".
Parents especially single parents worry will their ex claim that they are negligent, or will Child Protective Services allege neglect or inadequate guardianship for either situation. Since NY does not have a statute that states at what age there is a presumption of being appropriate, the answer is found by considering the factors for "it depends".
Clearly age is a factor but more important is maturity to answer the babysitting or "home alone" question. A mature, observant, responsible 12 year old might be fine compared to an irresponsible, distracted 15 year old. A child with special needs or developmental delays may need more sophisticated child care and likewise the same child regardless of age may not be capable of adequately watching out for a younger sibling.
Another factor will be the amount of time that the situation exists is it one hour while a parent has a doctor's appointment or is it all day while the parent is at work? What assistance is available for the child if needed? Is there a neighbor or older family member nearby that can assist if requested, how far away is the parent from the home, is a meal necessary, is there a medication schedule that has to be followed? Have there been problems in the past due to lack of supervision? Is the older child willing to watch the younger child? Is the older child watching an infant or watching a 10 year old sibling?
So the questions and situations are very unique and therefore the "it depends" answer is clearly apparent. A parent needs to try and avoid the Monday morning quarterback who claims that the parent should or could have known that it was inappropriate. The last thing any parent needs is to be in the middle of a case seeking to change custody or having CPS file a complaint. Good judgment is the most that you can bring to the question and if something went wrong but you still exercised good initial judgment any litigation against you may be unsuccessful but the obvious preference is to avoid being questioned or second guessed.