It seems as hard as I try, whenever playing with a band my index and middle finger of my plucking hand seem to stiffen up. Its not a cramp but it sure keeps it hard to play. It has nothing to do with age, I'm 18; still, it feels as if I'm moving exponentially slower as the songs get faster. And I am.
I've had this problem before aswell, and from what I think is going on it has to do with your muscles, when you get tense or nervous your muscles will begin to tense up as you know you are getting closer to the faster part and then you end up trying to hard to keep up making your muscle tense up alot and get really stiff, slowing you down and causing mistakes.
thats what I've noticed with my playing anyways, still happens with me, maybe thats more of a cramp then a stiffness, but it might be similar, one tip that I got that helps loosen me up when I play is pluck lighter and play louder (volume wise) it makes it alot easier to play and less chance of cramp and stiffening up
Stiff Fingers in Right hand 
