I'm 41. Traditionally, as you got older you had more demands on your time, but you could offset that with experience which made you faster and better. I don't know if I'm slower than I was in my 20's, but I definitely know that I spend a lot less time developing my skills outside of work than I did back then. I have two kids, a house that needs fixing, and a million other things to do.

I wonder if part of the reason so many IC's go in to management is simply that "how do you handle a team conflict" has a more evergreen answer than "how do you do some random task in X framework, describe the cost of different approaches in big-O notation, and do it fast". Or, of course, you've had 20 years to build up your network of professional friends and you all get each other the nicer jobs.