I think if you know the teen and she's responsible, doesn't matter about experience. A lot of people have no experience about things they do for the first time, a good dollop of common sense is all you need most first times.
this is how i feel - age is nothing but a number - if she is responsible and you trust her to take care of your children as you expect them to be cared for, go for it!
i was babysitting kids by the time i was 12, newborns by 14 (youngest was 5 weeks - brother was unwell and she needed to get him to the doctors, she was a very nervous mummy and just couldn't cope). all were within 5 minues drive for MY mum if things went to hell and i needed her (never did, but knowing she was available helped).

i think looking past the age and looking at the person is the ONLY way to decide who you are comfortable looking after your kids. make sure you are clear about your "rules" re bedtime (routine/drinks etc) and what your expectations are

i used to get $20 for an evening caring for a kidlet, up to 9.30pm, then i went to $10 an hour after that (mum did FDC so she made sure i didn't get ripped lol). during the day it was dependent on who, how many kids etc as to what we negotiated for me to be paid. most of the kids i looked after were FDC kids who became friends of the family, so the parents knew me, had seen me with their kids etc...