I think that building up strong habits around what you are working on makes it easy to maintain momentus.
I also agreee that even small increments of time can incredibly productive, like listening to an audiobook during a 15 minute commute.
It provides just enough information to interest you in a subject, then offers a few common theories with a reasoned most likely case (this is especially prevelant in the beginning of the book due to the lack of information available about humanity tens of thousands of years ago). Most of the assumptions made though are not the author's and there are references in the Notes section at the back if you do want to pursue these further.
Actually I followed that motto to build https://everydaycheck.com which is actually about that, to work on goals every day. One of the tricks I always share is to quantify the daily milestone, like "15 minutes of learning js" or "run 3km"
15 minutes per subject per day is a bit controversial for me though, as when I get into the swing of things, I work really hard. But I’ve never thought about this affecting momentum, perhaps my getting overwhelmed is actually a sign of burnout
I always remind myself "15 minutes a day is better than an hour every three days". But in order to prevent getting over-stressed about time and how quickly you're learning, you have to put your ego aside and simply stick to a schedule of learning a little bit of each subject every day.
That means limiting yourself when you want to go on a binge just as much as it means making time to get your 15 minutes in. Three days of binging on a subject can start to build pressure that you should maintain that level, and that's not sustainable.
Small amounts of new knowledge, every day, until you've hit your goal.
No matter how old you are, you still have plenty of time to take things slow.