I'm a 43 year old software engineer. I definitely do not have the ability to focus on coding (and mathematics) the way I did 20 years ago. It's difficult to describe why exactly, but it's sort of like my ability to reason formally is more prone to breaks in concentration and lapses. I can still do what needs to be done but it takes longer and I have less patience, especially for bit-twiddling.
At the same time, I think I'm a better programmer than I ever was. The programming abstractions I design are better for years of work with different systems. Also, not being able to fit everything in my head as easily helps me write code that is more modular with simpler pieces.
So I feel like I've lost the ability to do the trickiest stuff, but that is a small part of coding and I'll take the skills in overall design any day.
At the same time, I think I'm a better programmer than I ever was. The programming abstractions I design are better for years of work with different systems. Also, not being able to fit everything in my head as easily helps me write code that is more modular with simpler pieces.
So I feel like I've lost the ability to do the trickiest stuff, but that is a small part of coding and I'll take the skills in overall design any day.