As a brief aside, I tried to think of an example with women — where a woman over 30 behaves badly and is given the “oh, she’s just a kid!” treatment. I can’t think of one. Maybe Lena Dunham on occasion? I don’t watch Girls or follow her, but I vaguely remember something about her being young as explaining mistakes.
The way this youthanizing (great play on words, btw), worked with your examples is bad, but I see it show up in more quietly sinister ways, too. I see a lot of men (not boys, men), who play the “but I don’t know how to do it!” card. It’s not an excuse to get out of trouble, really, but a pre-set response to being asked to do something they don’t want to do. Generally speaking, that thing they’ve been asked is something that is traditionally feminine in nature — though not always. It’s maddening. Why do you think I know how to fold a fucking fitted sheet because I’m a woman??? (for example)
It’s all part of the limitless chances culture that surrounds young white (particularly middle class and higher) men.