Its not about the age of the engineers but maturity. Some just dont care about quality of their work because they get paid either way. Look at Silicon Valley, "ageism" is real there. They need young devs with ideas and huge skill to bring them to life, to stay ahead of the competition. Most companies dont understand that and blindly try to copy that often because their management is not competent enough.

Reasons for current situation are plenty. World and ppl are complicated.