I think that people have a tendency to set too much store by the characters or histories of famously brilliant scientists.
As it happens, Witten certainly wasn't able to write a PhD thesis at age 15. He was a history major, who developed an interest in maths/physics a couple of years after he graduated. Would
you recommend this as a career path to someone who wanted to be a Fields medallist?
Hawking's start in academia was marred, by his own admission, by chronic laziness. He estimates that he did about 1,000 hrs work in total in his whole degree- for a bachelor's degree from oxford, that works out at about 3 hours work a day for each working day (mon-fri) of term time. His exam results were barely good enough for him to be awarded a first, and he had to do a viva to get it. He only really started to work when he started to race the onset of his medical condition.
Lest you get the impression that no-one who achieved anything worked a day in their life, Newton spent most of his life as a recluse working alone in his room. Einstein was the best in his school at maths and physics, but didn't excel at university, and couldn't get a PhD place. His breakthroughs came by sitting at a desk in an easy job and thinking very, very clearly. Someone would really struggle to replicate that feat with the level of sophistication of physics today. Dirac studied electrical engineering at Bristol as an undergrad. He also said "A mathematically beautiful theory is more likely to be true than one that happens to fit some experimental data". Feynman, in complete contrast, said "It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong." Heisenberg was consistently brilliant in virtually everything he did- be it maths, skiing, or playing Beethoven concertos on the piano- but nearly failed his PhD exam because he hadn't bothered to make an effort to master experimental techniques and concepts. Landau worked himself into such fatigues that he couldn't sleep, and strove to master as much of physics as possible, hence the breadth of his course in theoretical physics with Lifschitz- covering everything from QED to the elasticity of solids.
Some famous physicists are simply brilliantly intelligent; some are profoundly original thinkers; some were maverick rebels who were in the right place at the right time for someone crazy to hit upon something true. All their stories are historically interesting, but I wouldn't draw any morals from them.