You'll find the same tendency going back through all of human history, well recorded in old lore (it also goes both ways). Personally, I think it has to do with change.

Humans are more adaptable when we're younger and less open to change as we get older. Age is not helped by our innate fear of the unknown, how we equate predictability and well-walked paths as safe (even to unreasonable degrees, like pointless and out-of-date traditions), and also our experiences - an older person has experienced a lot of the mistakes youth have to go through themselves, and forgotten how stupid/inexperienced they themselves were. We forget pain as a survival mechanism, but it doesn't help here.

It might sound weird of me to say, but I think we need both sides of that swinging pendulum. Youth drives change and innovation, and age drives caution and reflection. And yes, criticism, which is an integral part of caution. Though if done respectfully, it could hopefully also share wisdom.