I have a 14 yr old. We still have her favorite high schooler come over once a week because she also helps with homework, great mentor, etc. If we are only going to be gone a couple of hours, we leave daughter home with a friend or alone to watch the dogs, and pay daughter.
I guarantee a sitter $40 no matter what (meaning if I cancel, or just stay out an hour or so) because she blocked off her night for me. After about 3 hours, I start adding $10 and hour to what I will pay her (CASH). Typically a night for us is anywhere from minumum $60 to $80 for the night.
My daughter does babysit for some neighbors and she has been paid $10 and hour. This is for 1 family with twin 4 yr old girls and a 9 yr old and another family has 3 girls 6, 4, and 3.
At 14, it sounds like they will be hanging out and playing at your home. Make sure to specify if cooking is off limits. My daughter LOVES the kitchen (just like me)!