If you don't remember September 11th, if you have to ask me "what year was that?" or can't tell me what you were doing, you are young. That means, anyone under say 18 years old is young.
I'm 30 and starting to feel my age. When I listen to popular music, I think "God, this is terrible! Say something worthwhile!" I look at the ages on alcohol or a pack of smokes (1993 and 1996) and I think about how many things I have in my apartment older than someone who can buy alcohol. I will freak out in 4 years when there are adults, 18-year-olds, that don't remember the 20th century.