Through the 20s and 30s, there's change, of course, but it's pretty gradual. Metabolism slows a bit. Some lines in the face, that kind of thing. Nose hair, of course. A few introductory gray hairs.
Around 40, it accelerates: metabolism drops again (clunk), the gray hairs go from being token residents to being a strong voting bloc, and I presume the stray hairs springing up where they aren't wanted are going to be kicking into high gear. Presbyopia is also kicking in, all of a sudden, which is a real annoyance. And I'm guessing it's all this change that (like in adolescents) makes people more prone to depression.
Anyway, I'm not depressed. Just observing. I had a sticky bun for breakfast this morning as a birthday treat, how could I be depressed?
In completely unrelated news, do you ever wonder what comedians would do if they could find anything funny about President Obama? How about if he could make jokes at his own expense? That, it turns out, would be awesome!