The answer to 10 or 11 ...

Could it really be this simple?  Do you think we refer to ourselves as two ages when we are citing an event in the past because the school system has been set up to encompass two age groups per year?

First grade -- 6-7 year olds

2nd grade   -- 7-8 year olds

3rd grade   -- 8-9 year olds

4th grade  --  9-10 year olds

5th grade  -- 10-11 year olds

6th grade -- 11-12 year olds

7th grade -- 12-13 year olds

and so on.  I don't know.  I think it is ingrained in our brains for whatever reason.  People say it ALL THE TIME.   I was 6 or 7.  10 or 11.  Definitely 12 or 13.

You never say I was 6 or 9.

I am just saying.