This is interesting as it shows what value people place on childcare.
Should it matter how old the person is, if the care is during the evening or during the day, or on the number of children? They are all certainly factors that could be considered when deciding what to pay for childcare but for the person doing the job, it is the same amount of time.
Most people are paid for the time they work, not for how productive they are in that work. Look at your own job... are you paid on an hourly basis or on a results basis, or perhaps a set fee for the project but the project length could vary?
With childcare it is all about time, isn't it? It is someone else's time caring for your child/children. This means having someone come to care for one child is expensive - if you have 3 or 4 children the cost may be more viable.
A teenager will often be happier with a lower amount, they value their time less. It does not mean they are any less capable at doing the job. When I cared for children as a teenager, I was just as capable as I am now in my 40's.
If you can find someone you trust who accepts less, then that is fine. If you have found someone you trust who wants a bit more then you go with that or you find someone else.