As one of the few left of my peers that still develops I would say that it is a milestone. There is a lot of burn out and churn along the way. I knew guys that left software dev to become a worm farmer. Most take the elevator up to management which is what I did becoming a CTO but I always position myself at small but growing companies so I could keep myself hands on with technology. I have two friends left that started dev around the same time as me (92). For most it's just not a long term industry and burnout usually takes it's toll.
That being said the web revolution happened in the early 90's and was a huge shift in development. There where a lot of older devs around at that time who where doing desktop and had no desire to make the jump so 40'ish is also kind of a demarcation line of where an epoch changed in development, so the fact that you see few over 40 devs could be a) that the web was small back then and there where only a handful of us doing it and b) The developers that where doing web in the early 90's where all young, therefore it is kind of origin of a new type of developer.
That being said the web revolution happened in the early 90's and was a huge shift in development. There where a lot of older devs around at that time who where doing desktop and had no desire to make the jump so 40'ish is also kind of a demarcation line of where an epoch changed in development, so the fact that you see few over 40 devs could be a) that the web was small back then and there where only a handful of us doing it and b) The developers that where doing web in the early 90's where all young, therefore it is kind of origin of a new type of developer.