A child at 12yrs old is old enough to not have a babysitter in the state of Florida

Yes, if they are mature enough and nothing happens.

It is not a issue of them staying alone unless there is a issue or something happens.

If the parents are divorced, this can be a issue fought over in court over custody, even though there are no 'laws" it is a matter of opinion.

The real problem is if something happens, if the house burns down, or a child gets hurt, then Children and family serivces will investigate and their opinion of the maturity of the child may not be the same as yours