To me, a person's chronological age has nothing to do with them being 'old' other than how they think about it. My great-grandmother still climbed trees and helped me execute elaborate practical jokes on our family at 76 years of age. My father gave up on life in his 50's, taking up have drinking, prescription drug abuse, hermitism, and was dead at 67-, a shell of the drill instructor who raised me. My Uncle Harold lived in his own independent living apartment until he was 96, trying to grab the ass of every health aide that came to check on him stealing stories about squurrel hunting with my mom 30 years earlier. I'm in my early 40's but still give kiddie valentines to coworkers, swan dive into a ball pit, and was more stoked about Disneyland than my kids. Now, when I wake up in the morning, my back insists that I'm a senior citizen-but my brain usually thinks we're somewhere between 25 and 5 years old. I hope I NEVER get old.
thenamestheygivearedumb also answered this question with: "Dinosaur."