What makes a person old?

Back in class last night and my lecturer was asking how many of us had experienced eating dinner with our families at least 6 out of 7 nights a week. The majority of the class put their hands up (ages range from 24 to late 30's) and the lecturer was shocked that so many of us had grown up eating one meal a day with our families sans the television. He really couldn't disguise his surprise as his comment to our raised hands was:

"That many of you? Really? Well I guess you're older."

Older than what?! I sat there thinking, "but I'm only in my late twenty's!". I'm a spring chicken. I still believe that I'm only aged 19. I guess I've started to slide into the 'older person' category.