What Happened to Jack?

I saw a picture of actor Jack Nicholson online today and it got me wondering just what the heck happens to some people as they age. Jack was a heartthrob when I was growing up, and while he wasn't my cup of tea exactly, I could see where he was handsome and quite attractive.

Now 77, he's turned into a monster and bears little resemblance to his former self. Thank goodness I have not undergone this transformation yet, at least not completely. But Jack is only 10 years older than I--is this what's in store for me? When will it happen? What happened to Jack anyway?