1. You wake up, and the first thought in your head isnt about work, sex, food...no...none of those.
The first thought in your head is: God, I can't wait to go to sleep tonight.
2. You are explaining slang/idioms to ESL students, and they ask for help defining what some slang they heard means...and you have never heard it before.
When ESL students are more hip than you...you're old.
3. If you can become sore from sleeping, you're old.
4. Begin any sentence at any time with: When I was your age...
5. When you try to bring up 80's pop culture, and the person you're talking to responds: "I think I learned about that in school" or "Oh yeah, I saw that on VH1's I love the 80's"