Interesting thought.
I would say that's because it's usually the first common experiences you have with strangers,
talking about common experience, when you say "...in 6th grade", you would feel like more people would relate to your story the same way.
Even though most people in the same grade are usually the same age, age becomes an individual and incidentally irrelevant matter when your identity in the school bases upon your grade and class regardless of your age.
In conclusion, we are more likely to find people using their grade as a time of reference when talking about their cool or funne experiences, thinking that more people can relate to it.
It would be less likely to find someone who, for example, when talking about their parent's death would say, "... died when I was in 6th grade" and would normally use their age as a reference to the time of the incident because it is an individual experience and they wouldn't expect anyone to relate to it the same way as they do.
