Reference valadity and duration?

I've noticed a lot of girls post policies that they won't take a reference more than 1 year old, and in some cases not even more than 6 months. If you are one of those girls, could you share what specifically you expect might change between 6 months and a year ago? For instance, do you think the gentleman gave up his former profession and instead decided to become an undercover cop during that time, etc... ? I was just wanting to get some feedback and details, if possible.