My unsuccessful learning experience was when I was 16 and starting my Maths A level. We had 2 teachers who split the course material between them, one taking pure maths and the other applied maths. The teacher in charge of the pure maths, which had enjoyed up to A level and received an A for (in the days before A* existed, so top marks in other words), was a young man who had just qualified from his PGCE and came to teach us A level maths.
The main problem wasn’t the teacher’s age, I’m sure plenty of young teachers are very capable, the issue was more that he didn’t have the self confidence to stand up to the more lively characters in the class. He didn’t know how to respond when a pupil would try and disrupt the class by talking about something irrelevant, like last nights television, therefore that pupil would carry on talking and valuable lesson time would be wasted. Even when he did ask people to be quiet a fair amount of disruption had already been made.
The other issue was that he didn’t know his subject, in this case pure maths, well enough. I remember clearly one day the entire class followed him working out a complicated equation, writing down his calculations as we went along. After using up two blackboards worth of calculations he came to his answer, only for one of the pupils to check the answers in the back of out text book and announce it was the wrong answer. By then he had erased half of his calculation and he couldn’t go back to see where he had made a mistake.
It wasn’t the only time his calculations were wrong or he was unsure of his subject, which made the pupils very nervous. The short term result was that my class was time-tabled extra maths lessons to try to cover for this teacher’s lack of covering his subject, with the other maths teacher working longer lesson hours to try to get us through the exam. The longer term result was that the Pure maths results for that year were far lower than expected and the teacher was moved down to teaching 11-12 year olds maths.