Well, as I am in my 50th year, I guess I should start applying some of these principles.
Your house plants are alive, and you can't smoke any of them.
Having sex in a twin bed is out of the question.
You keep more food than beer in the fridge.
6:00 AM is when you get up, not when you go to bed.
You hear your favorite song on an elevator.
You watch the Weather Channel.
Your friends marry and divorce instead of hook up and break up.
You go from 130 days of vacation time to 14.
Jeans and a sweater no longer qualify as "dressed up."
You're the one calling the police because those damn kids next door won't turn down the stereo.
Older relatives feel comfortable telling sex jokes around you.
You don't know what time Taco Bell closes anymore.
Your car insurance goes down and your payments go up.
You feed your dog Science Diet instead of McDonalds leftovers.
Sleeping on the couch makes your back hurt.
You no longer take naps from noon to 6 PM.
Dinner and a movie is the whole date instead of the beginning of one.
Eating a basket of chicken wings at 3 AM would severely upset, rather than settle, your stomach.
You go to the drug store for ibuprofen and antacid, not condoms and pregnancy tests.
A $4.00 bottle of wine is no longer "pretty good stuff."
You actually eat breakfast food at breakfast time.
"I just can't drink the way I used to," replaces, "I'm never going to drink that much again."
You no longer drink at home to save money before going to a bar.
You read this entire list looking desperately for one sign that this doesn't apply to you.