You can actually learn much faster and understand something if you like (interested) with it. At school, it's almost impossible to focus on the learning you love because the majority of the school system forces you to fill your brain with things, including things you do not like and do not care about.
That's because sometimes the school system is called passion killer and creativity. Many children lose their passion and creative spirit because of the demands of the school / parents that always lead to the exact things such as job stability, promising career prospects, job opportunities and the amount of salary earned.
I'm not saying it's a bad thing because surely there are also advantages and disadvantages.
Therefore I think the best time (golden age) for fast learning is at the age of 18-22 years (at college or before work), because nearly 50% of people who have worked never read a book at all. Actually they can still learn, but most of the people who work have been lazy to learn again because it is too busy with work or not have the motivation to learn. If you are still in school / college then try to learn something you like, look for your passion, because in addition you can learn faster, you still do not have much risk and burden so you really are free to experiment with your passion-passion.