At some point in your 40s, you start noticing things. Recovery from a hard workout stretches from one day to three. Your knees feel stiff on the stairs first thing in the morning. The skin around your eyes looks a little less firm than it did last year. These changes tend to sneak up on you, and most people chalk them up to "just getting older."
That explanation isn't wrong. But it's incomplete. A lot of what you're feeling traces back to a single structural protein: collagen. And once you understand what's happening to it after 40, the changes make a lot more sense.
