This is all very interesting reading.
Back when I was 17, I think I charged £1.50 per hour, possibly it was £2 per hour (sorry, I didn't keep notes!) Add on some 15-20 years inflation and I suspect that £5 per hour is probably not far off (anyone know the inflation figures for the past 20 years?).
At age 17, I was First Aid trained and was helping at Beavers & Cub Scouts... so back then I feel that £2 per hour was probably quite reasonable.
These days I charge a lot more... but also babysit less often. As a teenager I recall being out almost every night.
Looking after someone else's children and home is a very responsible job. Anything could happen, or not happen.
When you have found a reliable babysitter whom you trust, who your children know and trust, and whom you know won't tell you 1/2 hour before you go out that they are not coming... you pay them as well as you can. After all, you want them to keep coming back - you don't want to have to find another babysitter each time and get your children used to being cared for by someone else. That great babysitter will give you peace of mind... when you go out, you won't worry too much about your children, home, pets etc.
How much is that peace of mind worth? Very hard to put a price on it... everyone will have a different view as to what is reasonable (as is clear from reading this thread).
Some teenagers can be very capable, while others are not. Just like some adults - some have common-sense, others haven't a clue. Some people have childcare training, others do not... is one better than the other - not necessarily. Some people can be great at paperwork, exams and the such but fail miserably to interact with an upset toddler, or cope with a baby who is teething.
Finding a reliable childcarer whom you trust completely, who your child adores and who you know will be able to cope with whatever your child/children throw at them can be very tricky. Finding that ideal person in your price range can be even harder - but ultimately we all have a max price we will pay for someone... so if your ideal babysitter charges more than you can afford, you will need to keep looking.