My 20 year old self was much better at making fast, bad decisions. Not taking the time to think through the higher order effects of those decisions. That 20 year old self lacks all the MOOCs I have taken, all the life and professional experience, 300+ non-fiction books.
I think at 20 I would have been a better test taker if we both started a college class right now but that is about it. I am way "smarter" or "more intelligent" though at 54 vs that 20 year old self. It isn't even close.
IMO much is because we are blind to ET Jaynes Mind Projection Fallacy in this area. We think of "intelligence" as a physical property like mass that gets weaker with age like muscles or bones. It is just such a given in our thinking and language that we can't help it.