I'm not saying you're wrong and 43 is still fairly young (I'm only 35) so it's possible even if there was a difference that it could still be negligible and there's all sorts variables that go into how well people learn as they get older.
But you could be wrong. Can you really accurately track how well you learn on a day to day basis over a lifetime? Doubtful. Even if you could really only learn 85% as well as you did when you were 20 it would be a measurable difference objectively. But subjectively that decline would be so minimal from one day to the next you would have a hard time noticing the decline. And since you still feel sharp at 43, and you felt sharp at 20, well nothing has changed, even though it clearly has.
Also some people just lie to themselves about how capable they (still) are. If you've ever dealt with an aging relative who was losing their vision but resistant to giving up driving. Or in the beginning stages of dementia but was in denial about it. Human beings will generally do anything to avoid admitting declining abilities of any sort. Or they will temper an admission with a but of some sort. It's just part of the experience.
You bring up a point about experience. Knowledge and experience have value too, obviously. After all young children can learn faster than adults period, but they don't know anything yet, and that's a disadvantage. And being able to learn a little bit better isn't so attractive that you'd be willing to give up all your accumulated experience. I wouldn't either.
But I am sure I can share that experience with a 20 something with no knowledge about software development and they'll pick it up much faster than my 60 something mother with no knowledge and software development. And that's where the learning bit comes in. And learning one more thing in a subject you're already an expert in, isn't really a tall order. Starting from scratch, like most children or young adults have to do is a greater feat and put on the same playing field, in a subject where you really have no experience or expertise, you might find the competition a little stiffer than you remember it being than when you were a kid. Still at 43 you're still young, but at 53? 63? 73? At some point there will probably be some obvious differences in your abilities that just become more stark as you age, even if you fare amazingly well against your peers in the same age group.
Research that seems to support that people who spend a lot of energy on learning and thinking and doing mentally demanding activities do better cognitively in their old age. I hope it's true, and specifically I hope it's true in my case.
If age hasn't proven to be an impediment for learning yet I just wouldn't worry about it. And even if things get tougher to learn as you get older it's not boolean, you can still learn things if you put the effort in, if you still want to learn it. Being less capable != incapable. This isn't a scenario where any position other than #1 is tied with last place, or first place loser, or any other sort of trope you want.