I think one overlooked factor is communication. If the instructor doesn't explain what standard of competency and performance they are expecting, the student doesn't know if he/she is there.
Often the student thinks they are ready, but they can't maintain the centreline on a landing, they can't or won't hold a consistent speed for approach and wander all over the sky anywhere between 800agl and 1200agl on downwind.
When the performance criteria are laid out clearly and exactly, and a safe attitude to controlling the aircraft is taken, progress is made and everyone is happy.
I still think too much emphasis is placed on when you go solo, and sometimes unintentionally by a well meaning club member. "You've got how many hours and you're not solo yet?"
When you are competent and safe, you and the instructor should both be happy to let you loose.
As far as 20 hours minimum for RA-Aus cert, I tell people that is a minimum, and they should expect anything up to 40 hours!
Age has an influence, but often the maturity of kids works against them.
Bottom line, we are all individuals, students, instructors, young or old and there can be no one answer that suits us all.
By the way, love the stability angle Tony, so true, if we set the aircraft up and trim, we can relax and enjoy flying so much more.
One last point, I really work hard to make sure people are relaxed, you can't control an aircraft if you are choking the stick with a death grip!