At age 30, I noticed I couldn't skimp on sleep anymore and hangovers were much worse than in my 20's.

In my mid-30's my eyesight started to blur and I had to start wearing glasses.

At 40, my digestion isn't as good as it used to be and I take supplements. Also, it's harder to memorize things now, and I no longer have the option of missing workouts or daily stretching, because I notice it much more if I haven't done these things.