K.L.
How old are the children? Is there any chance that any of the children will wake up while you are gone? Is the babysitter experienced? CPR? First Aid?
In Maryland the age for a babysitter is 13. I took a babysitting course at my church when I was 11 (6th grade) and started babysitter for a family friend at that time. They lived less than a mile from my parents house. Generally I went over around 8, the parents would put the younger 2 in bed before they left (they were 18 mos and 3 yrs). Around 9 I put the older one to bed (she was 6). Then I would watch t.v., play online and talk on the phone until the parents came home, which was generally some time between midnight and 2 AM.
I was very mature for my age, and as I said, my parents were less than a mile away. Now, 14 years later, I am a full-time nanny and still babysitting nights and weekends! Its something I trully love and the money is pretty good.
Ultimately the decision is up to you. Use your judgement to determine what you are comfortable with.