As said above, no general rule for the solo...
I soloed after something like 16 hours, I was 15 years old (French regulation allows that).
As an instructor, I trained a lot of PPLs on an uncontrolled airfield, so I wanted to make sure they could divert if needed with no ATC assistance.
That's why I preferned no to hurry to let them solo, so never less than 12 for me, most of them more than 15.
We desagreed on that point with the chief pilot, he often endorsed students for solo with less than 10 hours.
I endorsed a bunch of students for the solo...
No average for the solo, it depended mainly on the intensity of their training, and on their age.
Don't think that the youngest the best, not always...
Young pilots are good at repeating exactly what you show them, and for maneuvers, but they are less good at making a decision because have a lack of maturity.
And sometimes you explain them something, they understand... but forget after 48 hours.
Older pilots need more time to understand and copy what you show them, but they understand more deeply, and are more calm, more mature, and make better decisions.
The one I'm very proud of : it was not his first solo, it was is first tailwheel solo that I endorsed.
But damn ! it took a while !
This nice guy was a former taxi driver in Paris. He had spent his life dreaming about flying.
He took his first flying lesson at the age of 62. I was not the instructor. He told me it took a while for him to get his PPL...
He remembered those pretty Pipercubs when he was a young boy, and there was one at the airclub, a L18 more exactly.
He really wanted to fly it.
He told me "you know I'm a bit old, I will probably need time to be able to fly it".
I answered "OK so we'll take time for that !"
I do not remember how many hours we flew together.
But if you had seen this smile when he taxied back after his "first" solo on the Piper... :-)
Frog