there are no guidelines to the age of the photo it self. But I guess a photo from the 80's or 90's will have a hard time getting in unless there are a lot of them. The photos choosen for the photo book need to fit to each other.
Only the WSP 2015/2016 photo book has a time guidline (photos need to be shot in 2015). For we have been making several street photo books already, it would be silly to keep on using "old" photos.
For the Public Transportation project is the first one with this theme, it is not a problem (yet).