Some notes from an older person: The best approach will vary based on topic, age, and constraints (e.g. professional with kids). When I was a teenager, I learned Calc I, II, III and a first course of differential equations. I retained most of it. When I decided to take more advanced math courses later on (just a few years later), I was always saddened by my inability to retain much after the course was over. I kept w…
Last year I taught myself a good deal of category theory, and you're right, five minutes is not really enough to learn most things - but it is enough time to start an exercise - and those five minutes often turn into something longer!