From all I've read, many bp's tend to be picky and will refuse one or the other because they've developed a preference.
Hard to say three of my bp's now will eat anything, one is a bit pickier, but she feeds regularly for me.
It's hard to say on the age thing, I used to concern myself with it to a certain degree, but now I can really only have a guestamite if I have some backround on said snake (which usually doesn't happen). So I never know.
Rusty