I don't have ages in mind specifically but I agree with maturity. Was just curious what thoughts were with this shooting of a 6 year old by his 11 year old brother while left alone.
I don't know your kids but, if the 11yr old is mature and the 6yr old isn't a problem child go for it. I threw out general safe ages that kids "better be mature enough" by.
But, if you raised your kids to be mature and responsible, you shouldn't run into any problems. After all, the only time it would be an issue is if something happened...