[ordu]

> I've definitely found that I could inhale information faster and memorize much faster as a teenager.

One of possible explanations is a passion. What really helps to memorize are emotions. If something triggers emotions or somehow connected to them, then you have much more probability of remembering it. I felt strong about mathematics as a teenager, any math result I found was a happy event. Or rather not any result, I felt nothing about trigonometry formulas and I struggled to remember them, I invented techniques to reconstruct them. Mostly those techniques had nothing to do with math. But the point is: I remember what I like and don't remember boring things. It is emotions at work.

> a much more developed sense of priority and focus in order to get more benefit out of less time.

Which is an evidence confirming my hypothesis: you are not as interested in knowledge you acquire as you are interested in results. You are juggling priorities and subject your learning process to some higher goals. No more learning driven by emotions, now rationality is the king, no place for emotions.

It doesn't mean that my hypothesis is true, I just mention it for a completeness: we don't really know what is the reason behind learning difficulties growing with age. AFAIK even neuroplasticity itself can be at least partially caused by emotions.