This may be changing somewhat as people live longer. The average age for nobel prize work is around 48, but many scientists are productive their whole lives.

Since most software development isn't anywhere near nobel or turing-class difficulty, age discrimination policies in hiring and firing make more sense in the context of other factors such as a desire for cheaper workers, trying to avoid retraining costs, and a desire for workers with minimal family or outside commitments (and no interest in work-life balance) as well as fewer health issues.