You can see the difference between a young (my first two pics) and a mature (the last pic) G. frondosa. The fronds really develop and expand in these. But, unless your cultivating them yourself, you run the risk of greater bacterial infection of the fruitbody, masses of dirt and bugs and other age related probs. I say if its close.... PICK IT!
And here is a related thread of a cousin, Grifola umbellata:
http://forums.mycotopia.net/showthre...light=frondosa (Grifola or contamination?)