At 37 I'm already beginning to feel it. I have to be careful with my back lest it decide to punish me (backs are quick to strain/injure and very slow to heal). Also, my knees no longer permit me to run more than about 3 times a week and I have to watch my diet closely because everything gives me reflux.

Not looking forward to 50 :( Unfortunately, like fusion power, the cure for aging seems to be perpetually "20 years away."